ahci: qoriq: Fix a compiling warning

kbuild test robot reports the warnings:
drivers/ata/ahci_qoriq.c: In function 'ahci_qoriq_hardreset':
>> include/asm-generic/io.h:163:2: warning: 'px_is' may be used
>> uninitialized in this function [-Wuninitialized]
drivers/ata/ahci_qoriq.c:70:14: note: 'px_is' was declared here
>> include/asm-generic/io.h:163:2: warning: 'px_cmd' may be used
>> uninitialized in this function [-Wuninitialized]
drivers/ata/ahci_qoriq.c:70:6: note: 'px_cmd' was declared here

This patch fixed it by introducing a local variable.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Tejun Heo <tj@kernel.org>