backlight: lp855x: fix initial brightness type

Valid range of the brightness is from 0 to 255, so initial brightness
is changed from integer to u8.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>