staging: dwc2: don't issue traffic to LS devices in FS mode

I fell over the problem reported in
https://github.com/raspberrypi/linux/pull/390

:

"Issuing low-speed packets when the root port is in full-speed mode
 causes the root port to stop responding. Explicitly fail when
 enqueuing URBs to a LS endpoint on a FS bus."

with my dwc2 testing in NetBSD, so I adapted the change to dwc2.

Signed-off-by: Nick Hudson <skrll@netbsd.org>
[paulz: fixed up the patch to compile under Linux, and tested it]
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>