f_gsi: Fix NULL pointer dereference when USB endpoint is not available

DPL USB GSI interface is not having out and interrupt (notify) endpoint.
Enabling DBG message with gsi_bind() results into NULL pointer
dereference while trying to access out endpoint or notify endpoint
related endpoint name. Fix this issue by checking out and interrupt
endpoint against NULL before accessing it.

Change-Id: I84b32c5af4d2b80a13571b74f15105fd8238d4aa
Signed-off-by: Mayank Rana <mrana@codeaurora.org>