sisfb: change register I/O functions to use fixed size types

Use fixed-sized types (u8, u16, u32) instead of plain C types.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>