staging: mt7621-pci: use gpio perst instead of builtin behaviour

Some boards seems to ignore the PERST builtin behaviour to properly
perform a pcie line reset. Use gpio PERST behaviour instead which
seems to be more common.

Fixes: bd1a05bd ("staging: mt7621-pci: use PERST_N instead of gpio control")
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>