backlight: lp8788: remove unnecessary platform_set_drvdata()

The driver core clears the driver data to NULL after device_release or
on probe failure, since commit 0998d063100 ("device-core: Ensure drvdata
= NULL when no driver is bound").

Thus, it is not needed to manually clear the device driver data to NULL.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>