USB: f_mtp: Check if the ep is not disabled before queuing it

Queuing a request on a disabled endpoint during composition switch leads
to prime failure. Hence return -EINVAL if a request is queued on a disabled
endpoint.

Also, in f_mtp, block queuing a request in OUT ep in receive_file_work, if
the device state is STATE_OFFLINE.

Change-Id: I0e706d5280a2460baf6ab05dbf97a09c59b642fb
Signed-off-by: Rajkumar Raghupathy <raghup@codeaurora.org>
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>