USB: gadget: mtp: Add module parameters for Tx transfer length

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

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

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

Also add provision for specifying no. of Tx requests at runtime.

echo 8 > /sys/module/g_android/parameters/mtp_tx_reqs

CRs-Fixed: 486455
Change-Id: I675fc3303be2aba081d2ab59a9efb94aa478f849
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>