backlight: generic_bl: use pr_info() instead of printk()

Use pr_info() instead of printk() to allow dynamic debugging.  The pr_fmt
prefix for pr_ macros is used.  Also fix checkpatch warnings as below:

WARNING: printk() should include KERN_ facility level

[akpm@linux-foundation.org: use KBUILD_MODNAME, per Joe]
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>