drivers/video/backlight/adp88x0_bl.c: fix bit testing logic

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

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>