usb: f_mtp: Limit MTP Tx req length to 16k for ChipIdea

Currently, the f_mtp driver marks the mtp_tx_req_len to 1M.
As a result, file transfer from device to host PC fails for
devices using ChipIdea controller since it does not allow request
length greater than 16k for IN Endpoint. Hence, to allow such
file transfers, limit Tx req length to 16k.

Change-Id: Ib9ca95880ca0b1a2f0c66817e54e9a7b2af53fe6
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>
[ppratap@codeaurora.org : replace DBG with mtp_log]
Signed-off-by: Pratham Pratap <ppratap@codeaurora.org>