DWC3: gadget: Report error if RX buffer not aligned to EP's MTU

Synopsys USB3 Controller (DWC3) has a restriction where size
of OUT requests (TRB) queued to the controller must be aligned
with the endpoint's max packet size. Hence, report error in case
function driver tries to queue an unaligned request.

Change-Id: Id5f4a0ff9d2f9c55bba936d683608c524eadd260
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>