ipmi: fix hardcoded ipmi device exit path warning

When modprobe.conf has
options ipmi_si type="kcs" ports=0xCA2 regspacings="4"

ipmi_si can be loaded properly, but when try to unload it get:

Sep 20 15:00:27 xx abrt: Kerneloops: Reported 1 kernel oopses to Abrt
Sep 20 15:00:27 xx abrtd: Directory 'kerneloops-1285020027-1' creation detected
Sep 20 15:00:27 xx abrtd: New crash /var/spool/abrt/kerneloops-1285020027-1, processing
Sep 20 15:01:09 xx kernel: ------------[ cut here ]------------
Sep 20 15:01:09 xx kernel: WARNING: at drivers/base/driver.c:262 driver_unregister+0x8a/0xa0()
Sep 20 15:01:09 xx kernel: Hardware name: Sun Fire x4800
Sep 20 15:01:09 xx kernel: Unexpected driver unregister!
Sep 20 15:01:09 xx kernel: Modules linked in: ipmi_si(-) ipmi_msghandler ip6table_filter ip6_tables ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat bridge stp llc autofs4 sunrpc cpufreq_ondemand acpi_cpufreq freq_table mperf xt_physdev be2iscsi iscsi_boot_sysfs bnx2i cnic uio cxgb3i iw_cxgb3 cxgb3 mdio ib_iser rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr ipv6 iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi dm_mirror dm_region_hash dm_log dm_mod vhost_net macvtap macvlan tun kvm_intel kvm uinput sg ses enclosure ahci libahci pcspkr i2c_i801 i2c_core iTCO_wdt iTCO_vendor_support igb dca i7core_edac edac_core ext3 jbd mbcache sd_mod crc_t10dif megaraid_sas [last unloaded: ipmi_devintf]
Sep 20 15:01:09 xx kernel: Pid: 10625, comm: modprobe Tainted: G        W   2.6.36-rc5-tip+ #6
Sep 20 15:01:09 xx kernel: Call Trace:
Sep 20 15:01:09 xx kernel: [<ffffffff810600df>] warn_slowpath_common+0x7f/0xc0
Sep 20 15:01:09 xx kernel: [<ffffffff810601d6>] warn_slowpath_fmt+0x46/0x50
Sep 20 15:01:09 xx kernel: [<ffffffff812ff60a>] driver_unregister+0x8a/0xa0
Sep 20 15:01:09 xx kernel: [<ffffffff812ae112>] pnp_unregister_driver+0x12/0x20
Sep 20 15:01:09 xx kernel: [<ffffffffa01d0327>] cleanup_ipmi_si+0x3c/0xa7 [ipmi_si]
Sep 20 15:01:09 xx kernel: [<ffffffff81099a60>] sys_delete_module+0x1a0/0x270
Sep 20 15:01:09 xx kernel: [<ffffffff814b7070>] ? do_page_fault+0x150/0x320
Sep 20 15:01:09 xx kernel: [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
Sep 20 15:01:09 xx kernel: ---[ end trace 0d1967161adcee0d ]---

We need to check if ipmi_pnp_driver is loaded before we try to unload it.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>