usb: dwc3: Update soft-reset wait polling rate

Starting from DWC_usb31 version 1.90a and later, the DCTL.CSFRST bit
will not be cleared until after all the internal clocks are synchronized
during soft-reset. This may take a little more than 50ms. Set the
polling rate at 20ms instead.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>