usb: gadget: mtp: Increase RX transfer length to 1M

The test results indicate that larger transfers improve write speeds.
The Rx request transfer length is now 16K bytes. Increase the Rx
transfer length from 16K to 1MB to get good throughputs.

Change-Id: I3c64e6b96d569b4dc61805a53a0835db9142d55e
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>