backlight: Declare backlight_types[] const

Since backlight_types[] isn't modified, let's declare it const.  That
was probably the intention of the author of commit bb7ca747


("backlight: add backlight type"), via which the "const char const *"
construct was introduced.  The duplicate const was detected by sparse.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>