8139too: Make PIO/MMIO a runtime decision through a module parameter.

This is needed to support devices that only work with PIO without
penalising devices that work fine with MMIO in distro kernels.
It also allows us to eventually use PIO as a fallback when setting
up MMIO fails.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>