f_gsi: Pass 64 byte as max_pkt_size to IPA driver

USB GSI function driver needs to pass max_pkt_size to IPA driver
based on enumerated USB speed. This change set max_pkt_size as
64 byte instead of 512 byte for full speed mode. It also updates
same value with IPA driver to program required scratch register
for GSI firmware use.

Change-Id: Ia43c2b743a6d5b018053bc822f6d3c757d7a4fb5
Signed-off-by: Mayank Rana <mrana@codeaurora.org>