Revert "mt7620: gsw: make IntPHY and ExtPHY share mdio addr 4 possible"

Revert "mt7620: gsw: make IntPHY and ExtPHY share mdio addr 4 possible"

This reverts commit b40316c21a960d332bc9b04ee1791b8aafcf8786.

That change causes ramips/mt7620 to fail with:

    drivers/net/ethernet/mtk/gsw_mt7620.c: In function 'mt7620_hw_init':
    drivers/net/ethernet/mtk/gsw_mt7620.c:171:14: error: 'mdio_mode' undeclared (first use in this function); did you mean 'd_move'?
      } else if (!mdio_mode) {
                  ^~~~~~~~~
                  d_move

Back it out for now to restore compilation.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>