dwc3: gadget: Ignore suspend event if USB speed is unknown

In some of host machine reboot usecases, it has been found that
multiple suspend events are being received before usb device is
being enumerated with host including receiving suspend before
BUS RESET. This suspend event is being seen due to not host
initiated suspend but due to LINE state at that point and may
put USB Host and Device out of sync where host may not send any
more BUS RESET which would cause USB device enumeration as unknown
device with host machine. Fix this issue by honouring suspend
event only when USB enumeration speed is known.

CRs-Fixed: 717151
Change-Id: I201f106d1be71716eaa7a6246d2187889ea53a9c
Signed-off-by: Mayank Rana <mrana@codeaurora.org>