f_gsi: Update max_pkt_size based on speed instead of using ep desc value

USB endpoints are enabled and disabled from interrupt context whereas
IPA and GSI related operations are performed in work context. Commit
41d5c6f1 "f_gsi: Pass 64 byte as max_pkt_size to IPA driver") used
endpoint related descriptor information to update max_pkt_size which
is resulting into NULL pointer dereference when there is race condition
happening between EVT_CONNECT_IN_PROGRESS event based operations and
USB endpoint being disabled from gsi_disable(). Hence fix this issue
by updating max_pkt_size based on speed instead of using value from
endpoint descriptor.

Change-Id: Ide82c2542674f25e8ebfd3d5cd0e77592d3611d4
Signed-off-by: Mayank Rana <mrana@codeaurora.org>