CAIF: fix sparse warning for caif_usb

This fixes the following sparse warning:
net/caif/caif_usb.c:84:16: warning: symbol 'cfusbl_create' was not
declared. Should it be static?

Signed-off-by: Silviu-Mihai Popescu <silviupopescu1990@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>