usb: gadget: f_mtp: Make RX buffer size 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. Generally, MTP userspace
module submits RX requests aligned to 512 bytes which works
fine in High speed mode. But, fails in Super-speed mode where
max packet size can be 1024 bytes.
To overcome this just change the size of RX request buffer
while submitting to DCD as the buffers are already allocated
with the size of 16KB.

Change-Id: I68c32ef3d08b2cf943b52a7269785d0a00146953
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>