usb: gsi: Add NULL pointer check

Static code analysis tool is detecting a possible NULL pointer
dereference for gsi->d_port.in_ep in the error handling path.
This change adds a NULL pointer check to avoid this issue.

Change-Id: I4adf03e933cece0698cd3761c8758f4bd98925f4
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>