usb: dwc3: gadget: driver should not wait for RxFIFO to drain

An older version of the databook said to wait for the FIFO to
drain, but that has been removed from the newer databooks.

Waiting for RxFIFO to drain caused problems when testing against
one of the host controllers available in the market.

After talking to one of the RTL engineers, he stated that we
should _not_ wait for RxFIFO to drain.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>