net: cdc-phonet: Staticize usbpn_probe

'usbpn_probe' is referenced only in this file. Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Rémi Denis-Courmont <remi@remlab.net>
Cc: Rémi Denis-Courmont <remi@remlab.net>
Signed-off-by: David S. Miller <davem@davemloft.net>