usb: gadget: f_mtp: Increase default TX buffer size

Increasing buffer size from 16k to 1MB improves throughput
from 36MBps to 77MBps with 8 usb request buffers. Hence add
default TX buffer macro of 1MB size and try to allocate the
same.

Change-Id: Ifebf5cbe46e24ae019829db388550d47fd4a5ff4
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>