net: fix cdc-phonet.c dependency and build error

Fix build error caused by missing Kconfig dependency:

ERROR: "cdc_parse_cdc_header" [drivers/net/usb/cdc-phonet.ko] undefined!
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/Kconfig

@@ -541,7 +541,7 @@ config USB_NET_INT51X1
 
 config USB_CDC_PHONET
 	tristate "CDC Phonet support"
-	depends on PHONET
+	depends on PHONET && USB_USBNET
 	help
 	  Choose this option to support the Phonet interface to a Nokia
 	  cellular modem, as found on most Nokia handsets with the