usb: storage: isd200: fix sparse warning

Fix the following sparse warning:

| drivers/usb/storage/freecom.c:122:22: warning: symbol
|	'freecom_usb_ids' was not declared. Should it
|	be static?

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