USB: make drivers/usb/input/wacom_sys.c:wacom_sys_irq() static

This patch makes the needlessly global wacom_sys_irq() static.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Ping Cheng <pingc@wacom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>