leds: renesas-tpu: cleanup a small type issue

Static checkers complain that, although this is declared as an
unsigned long, we can only use the lower 32 bits.  For anything
higher, we hit bugs widening then bitwise negate or wrapping bugs
doing the left shift.

From looking at the context, this is not a problem because we only
use 16 bits.  I've changed some types to make it more clear.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>