Fix the following compile warning:

drivers/usb/serial/kobil_sct.c: In function ‘__check_debug’:
drivers/usb/serial/kobil_sct.c:719:1: warning: return from incompatible
pointer type [enabled by default]
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>