staging: mt7621-pinctrl: remove 'rt2880_pinctrl_pin_dbg_show' callback

The debug information provided by this function does not make
sense at all, so just remove it.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>