usb: dwc2: Change TxFIFO and RxFIFO flushing flow

Before flushing fifos required to check AHB master state and
lush when AHB master is in IDLE state.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>