USB: gadget: mtp: 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: I74d5da09437c25e9f1772f7cd49aecb8e4fedcf2
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>