usb: dwc3: Refactor dwc3_mode_show

Use dwc3_mode_string as the single source of truth for mode-to-string
conversion.

Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20250822092411.173519-2-khtsai@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>