backlight: lp855x_bl: simplify bl_get_brightness()

Getting the brightness value is not critical, no need to read the actual
register value.  To simplify it, just return the 'bl->props.brightness'
value.  Then, lp855x_read_byte() can be removed, not used any more.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-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>