usb: gadget: mtp: Dequeue request upon vfs_write failure

When vfs_write fails, a request was already queued
on the out endpoint. If the request was rx_req[0], 
mtp_read results in a failure since it tries to queue
the same request. Fix this by dequeuing the request
when there is a vfs_write failure.

Change-Id: If82d9f7f3fb496cdc27983bd306d61000dc3396d
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>