usb: gadget: rndis: fix broken data aggregation build

Declare "cdev" to fix the 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.

Change-Id: I01f81a2b00f55f381b187434a59d74efb2b87908
Git-Commit: 769c58c0565cb28c8cd462ccfc065f12854889b1
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.9


Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>