usb: f_gsi: Fix error handling path in ipa_connect_channels

Commit 7674e8f6 ("usb: f_gsi: Don't enable IPA data path
if connect channel fails") added error handling path and IN and
out endpoints. For DPL out ep and out request are NULL. In case
ipa_connect_channels returns error for DPL usb_gsi_ep_op() is
de-referencing NULL pointer. Add NULL check for out ep in error
handling path.

Change-Id: Ia2db2ed840a4ab9032366042f00a1a9ac086d91f
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>