USB: f_mtp: Set maximum burst value to two for superspeed mode

Currently burst value is set to zero for superspeed in mtp driver.
MTP throughput can be improved by setting max burst vlaue to non
zero value in superspeed mode. Hence set max burst vlaue to two
for both IN and OUT bulk endpoints in superspeed mode.

CRs-Fixed: 532070
Change-Id: Ib78b8fec5d1ab934f4d4ec80b7e008149707ce0f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>