usb: f_gsi: Fix typecasting of usb_prot_id

usb_prot_id enum extends ipa_usb_teth_prot enum for
various USB functions using F_GSI driver. Perform
appropriate type-casting when using usb_prot_id as
ipa_usb_teth_prot.

Change-Id: Ie148bc5038e0de3fc0056d9f61ebf1c12185a8d1
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>