regulator: lp8755: Fix mask for pchip->mphase

According to the datasheet, it has 9 multi-phase mode from 0 to 8 and it takes
4 bits in the register.
The mask for pchip->mphase should be 0x0F.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>