caif_dev: fix sparse warnings for caif_flow_cb

This fixed the following sparse warning:
net/caif/caif_dev.c:121:6: warning: symbol 'caif_flow_cb' 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>