ANDROID: usb: gadget: f_accessory: assign no-op request complete callbacks

ANDROID: usb: gadget: f_accessory: assign no-op request complete callbacks

The req->complete seems to presist the callback pointer for
the control requests. This causes the serial of the accessory
to be overridden when an accessory function specific out
control request is issued right after the  ACCESSORY_SEND_STRING
control request. Therefore, assign a no-op req complete function
when nothing needs to be done once the request is completed.

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Bug: 63867169
Change-Id: I78c1602e9a044b8718b270b8a068cf5afc83f984