[ARM] fix pxa930_trkball build errors

drivers/input/mouse/pxa930_trkball.c: In function `pxa930_trkball_probe':
drivers/input/mouse/pxa930_trkball.c:189: error: `ret' undeclared (first use in this function)
drivers/input/mouse/pxa930_trkball.c:230: error: `ret' undeclared (first use in this function)

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>