drivers/video/backlight/lcd.c: return ENXIO when ops functions cannot be called

Previously, when ops functions cannot be called, lcd_show_contrast() and
lcd_store_contrast() returned 0, instead of ENXIO.

Thus, in this case, a local variable 'rc' for return value should be
initialized as ENXIO, after kstrtoul() is called.

Signed-off-by: Jingoo Han <jg1.han@samsung.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>