USB: gadget: Add module parameter for Rx transfer length

The Rx request transfer length is 16K bytes by default.  The test
results indicate that larger transfers improve write speeds.  Add
provision for specifying Rx transfer length at runtime.

echo -n 1048576 > /sys/module/g_android/parameters/mtp_rx_req_len

The above command can be used to set Rx transfer length to 1MB.  If
the memory allocation is failed, fallback to the default length.

CRs-Fixed: 429212
Change-Id: I7bed5aeefabf1a50c08a9a8e5b876e0cf59515fd
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>