usb: dwc3: Skip check for U3 when handling Function Suspend

Skip check for U3 link state when we receive Function Suspend.
Link is not in U3 when host sends Function Suspend, but will
be in U3 if/when the subsequent Bus Suspend arrives.
If we check for U3 in the case of Function Suspend, we will
not disconnect channels with IPA as the API that checks
pre-conditions for suspend will always return FALSE.
The U3 check is done as usual in the case of Bus Suspend from
the host.

CRs-Fixed: 966352
Change-Id: Ie9ee045a096b58256d2e8814338ffe0aa3c01163
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>