staging: mt7621-pci: simplify read_config function

read_config function is always called with bus and func
being 0. Avoid those params and just use 0 inside the
function. Return readed value instead pass a reference
parameter.

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