b43: Fix machine check error due to improper access of B43_MMIO_PSM_PHY_HDR

Register B43_MMIO_PSM_PHY_HDR is 16 bit one, so accessing it with 32b
functions isn't safe. On my machine it causes delayed (!) CPU exception:

Disabling lock debugging due to kernel taint
mce: [Hardware Error]: CPU 0: Machine Check Exception: 4 Bank 4: b200000000070f0f
mce: [Hardware Error]: TSC 164083803dc
mce: [Hardware Error]: PROCESSOR 2:20fc2 TIME 1396650505 SOCKET 0 APIC 0 microcode 0
mce: [Hardware Error]: Run the above through 'mcelog --ascii'
mce: [Hardware Error]: Machine check: Processor context corrupt
Kernel panic - not syncing: Fatal machine check on current CPU
Kernel Offset: 0x0 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffff9fffffff)

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [2.6.35+]
Signed-off-by: John W. Linville <linville@tuxdriver.com>