powerpc/eeh: Don't release eeh_mutex in eeh_phb_pe_get

Acked-by: Gavin Shan <shangw@linux.vnet.ibm.com>

=====================================
[ BUG: bad unlock balance detected! ]
3.6.0-rc5-00338-gcaa1d63-dirty #6 Not tainted
-------------------------------------
swapper/0/1 is trying to release lock (eeh_mutex) at:
[<c000000000058218>] .eeh_add_to_parent_pe+0x318/0x410
but there are no more locks to release!

other info that might help us debug this:
no locks held by swapper/0/1.

stack backtrace:
Call Trace:
[c00000003e483870] [c000000000013310] .show_stack+0x70/0x1c0 (unreliable)
[c00000003e483920] [c0000000000d8310] .print_unlock_inbalance_bug+0x110/0x120
[c00000003e4839b0] [c0000000000d9a50] .lock_release+0x1d0/0x240
[c00000003e483a60] [c000000000778064] .__mutex_unlock_slowpath+0xb4/0x250
[c00000003e483b10] [c000000000058218] .eeh_add_to_parent_pe+0x318/0x410
[c00000003e483bc0] [c00000000005a118] .pseries_eeh_of_probe+0x258/0x2f0
[c00000003e483cc0] [c000000000032528] .traverse_pci_devices+0xa8/0x150
[c00000003e483d70] [c000000000aa7288] .eeh_init+0xd4/0x140
[c00000003e483e00] [c00000000000abc4] .do_one_initcall+0x64/0x1e0
[c00000003e483ec0] [c000000000a90418] .kernel_init+0x1e8/0x2bc
[c00000003e483f90] [c00000000002048c] .kernel_thread+0x54/0x70
EEH: PCI Enhanced I/O Error Handling Enabled

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>