drivers/video/backlight/adp8860_bl.c: fix ambient light zone overwrite handling

This affects the get/set of the current Ambient Light Zone.  Reading
should return an integer between 1..3 (1 = Daylight, 2 = office, 3 =
dark).  Writing a value between 1..3 forces the backlight controller to
enter the corresponding Ambient Light Zone.  Writing 0 returns to normal
operation.

Fix valid range checking so we don't write invalid values to the
controller, and make sure we subtract 1, since this is what the register
definition (CFGR:BLV) requires.  Otherwise the values written don't work
correctly.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Richard Purdie <rpurdie@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>