usb: dwc3: Set vbus_active in peripheral-only mode

The recent change in commit 904ab34b ("usb: dwc3: ep0: Fail
enqueue() when racing with vbus_sess disconnect") added a check
to prevent queuing on ep0 if vbus_active is unset. However on
some configurations if dwc3 is configured as peripheral-only, this
flag is not set. In those cases, simply set vbus_active to true.

Change-Id: Iaf8612b924ead1484dc585377e7eb0a46cc89a16
Fixes: 904ab34b ("usb: dwc3: ep0: Fail enqueue() when racing with vbus_sess disconnect")
Signed-off-by: Jack Pham <jackp@codeaurora.org>