backlight: da903x_bl: use dev_get_drvdata() instead of platform_get_drvdata()

dev_get_drvdata() can be used instead of platform_get_drvdata()
to make the code smaller.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>