backlight: atmel-pwm-bl: fix reported brightness

The driver supports 16-bit brightness values, but the value returned
from get_brightness was truncated to eight bits.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>