misc: pmic: fix regression in pmic_fsl.c (SPI)

This fixes write access to PMIC registers, the bug was
introduced partly in commit 64aac65099 and in commit c9fe76dd91.
It was tested on an i.mx31 with a mc13783.

Signed-off-by: Helmut Raiger <helmut.raiger@hale.at>
Acked-by: Stefano Babic <sbabic@denx.de>