staging: mt7621-pci: use PERST_N instead of gpio control

Driver is using gpio control instead of each port PERST_N
bit. Use PERST_N bit of each port cleaning all the ugly
gpio code in driver probe function.

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