backlight: adp8860: Fix another uninitialized variable use

A recent patch I did fixed two potential uses of uninitialized
variables in the adp8870 and adp8860 drivers. Unfortunately,
I missed another one:

drivers/video/backlight/adp8860_bl.c: In function 'adp8860_bl_ambient_light_level_show':
drivers/video/backlight/adp8860_bl.c:570:11: warning: 'reg_val' may be used uninitialized in this function

This does the same change as before in one additional function,
and also changes the check for the return value in a way that
avoids another false positive warning with a similar message.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 6be3a5a9cd91 ("backlight: adp88x0: Fix uninitialized variable use")
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>