usb: dwc3: debugfs: Print/set link state for peripheral mode

Current implementation only prints/sets the link state for peripheral
mode only. Check and prevent printing bogus link state if the current
mode of operation is not peripheral.

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