usb: gadget: throttle IRQ rate for SuperSpeed

There was a merge error from commit 6e0c86d1 "USB: gadget:
u_ether: Fix data stall issue in RNDIS tethering mode"
that resulted in the accidental removal of checking if
the gadget is connected at SuperSpeed. Re-introduce this
check so that IRQs on the downlink path are throttled,
decreasing the load on the CPU.

Change-Id: Ic2aa1d433e0fded95c6e825a760e89f726360522
Signed-off-by: Jack Pham <jackp@codeaurora.org>