ANDROID: usb: gadget: f_accessory: Mitgate handling of non-existent USB request

Prevents mishandling USB requests that are no longer present.

Bug: 161010552
Fixes: 483cb562 ("ANDROID: usb: gadget: f_accessory: Add Android Accessory function")
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Change-Id: I8ff24d6d49214c3bd10a1b5d5e72814ec2a91c61