staging: mt7621-pci-phy: add 'mt7621_phy_rmw' to simplify code

In order to simplify driver code and decrease a bit LOC add new
function 'mt7621_phy_rmw' where clear and set bits are passed as
arguments.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200315160154.10292-1-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>