usb: serial: kobil_sct: fix compile warning:

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: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>