staging: dwc2: use dwc2_hcd_get_frame_number where possible

Before, there were two places that manually read the FRNUM registers,
while there is a function to do this.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>