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>