mfd: Make pcf50633_write_block() return 0 on success

The callers of pcf50633_write_block assume pcf50633_write_block return 0 on
success, thus make it return 0 instead of the number of registers written on
success.

Currently pcf50633_write_block is called in drivers/mfd/pcf50633-irq.c and
drivers/rtc/rtc-pcf50633.c.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>