e100: Optionally use I/O mode only to access register space

It appears that some systems still like e100 better if it uses
I/O access mode.  Setting the new parameter use_io=1 will cause
all driver instances to use io mapping to access the register
space on the e100 device.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>