staging: mt7621-pci: simplify write_config function

write_config function is always called with bus and func
being 0. Avoid those params and just use 0 inside the
function. Review parameter types changing for more proper
ones.

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