usb: gadget: mbim: Enable wMTU parameter value to be runtime configurable

Adding option to configure the mtu parameter from user space according
the use requirements. The default value is set to 1500.

CRs-Fixed: 503003
Change-Id: I52a483d62e72cf4cce0309754b1fe8e73130c75d
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>