staging: mt7621-pci: handle phy using new generic phy 'pci-mt7621-phy' driver

Phy part of this driver has been moved to a new 'pci-mt7621-phy' driver.
Change code to use kernel's generic phy API removing also no more need
moved code.
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>