staging/xgifb: Remove unnecessary casts

Both functions xgifb_reg_get and inb return an u8 value, so we don't
need to cast their return value to unsigned char.

-> remove the cast

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>