mfd: Set tps6586x bits if new value is different from the old one

mfd: Set tps6586x bits if new value is different from the old one

It does not make sense to write new value only when all the bit_mask
bits are zero.
We need to write new value if the bit mask fields of new value is
not equal to old value.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>