usb: gadget: rndis: fix broken data aggregation build

Declare "cdev" to fix broken AOSP commit
"RNDIS: Add Data aggregation (multi packet) support",
otherwise we run into following build failure:

  CC      drivers/usb/gadget/function/f_rndis.o
drivers/usb/gadget/function/f_rndis.c: In function ‘rndis_command_complete’:
drivers/usb/gadget/function/f_rndis.c:479:3: error: ‘cdev’ undeclared (first use in this function)
drivers/usb/gadget/function/f_rndis.c:479:3: note: each undeclared identifier is reported only once for each function it appears in
make[4]: *** [drivers/usb/gadget/function/f_rndis.o] Error 1

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>