usb: dwc2: debugfs: Don't touch RX FIFO during register dump

Dumping the registers via debugfs makes USB on Raspberry Pi completely
unusable. The read of register GRXSTSP ("Receive Status Read and Pop
Register") is responsible for this behaviour, because it pops the RX FIFO.
So avoid this by omitting the relevant register.

CC: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Fixes: 563cf017 ("usb: dwc2: debugfs: add support for complete register dump")
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>