pata_legacy: wait for async probing

The basic problem here that pata_legacy attaches the host, sees if it found
any devices and detaches it if none were found.  With async probing, it's not
waiting until discovery is finished before deciding it has no devices and
trying the detach leading to this warning:

ata1: PATA max PIO4 cmd 0x1f0 ctl 0x3f6 irq 14
------------[ cut here ]------------
WARNING: at drivers/ata/libata-core.c:6222 ata_host_detach+0x75/0x90()
Modules linked in:
Pid: 1, comm: swapper Not tainted 2.6.30-rc7 #1
Call Trace:
 [<c01fbb05>] ? ata_host_detach+0x75/0x90
 [<c01fbb05>] ? ata_host_detach+0x75/0x90
 [<c01139b5>] ? warn_slowpath_common+0x45/0x80
 [<c01139fa>] ? warn_slowpath_null+0xa/0x10
 [<c01fbb05>] ? ata_host_detach+0x75/0x90
 [<c02f40e0>] ? legacy_init+0x44e/0x87f
 [<c02f3c92>] ? legacy_init+0x0/0x87f
 [<c0101021>] ? _stext+0x21/0x140
 [<c01890ff>] ? proc_register+0x2f/0x190
 [<c018938c>] ? create_proc_entry+0x5c/0xc0
 [<c0135ebe>] ? register_irq_proc+0x6e/0x90
 [<c02e6484>] ? kernel_init+0x6e/0xbf
 [<c02e6416>] ? kernel_init+0x0/0xbf
 [<c01031d7>] ? kernel_thread_helper+0x7/0x10
---[ end trace ef1ee36e873ae3a0 ]---

Because it detaches before the probe is complete.

One way to fix it would be to put an async_synchronize_full() before looking
for devices, which this patch does.  A better way might be to separate libata
into its own domain and only wait for that.

Reported-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>