mfd: sta2x11: Fix potential NULL pointer dereference in __sta2x11_mfd_mask()

The dereference to 'mfd' should be moved below the NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>