mfd: Set adp5520 bits if new value is different from the old one

Current code checks if all the bit_mask bits are all zero is wrong.
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>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>