usb: gadget: f_gsi: Add MaxPacketSize in fs_descriptor

Currently "wMaxPacketSize" is not set in full speed descriptor
for MBIM and ECM functions. This results in chapter 9 test failures
on MBIM and ECM compositions. Hence updated Maxpacketsize in full speed
descriptor.

Change-Id: I0e73720919fdfa785c0bd456531a0d68516af36d
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>