e1000e: restore flow control settings properly

After a cable unplug the forced flow control settings were lost
accidentally and the flow control settings fell back to the default
EEPROM determined values. This breaks for people who want to
run without fc enabled - after a cable reset the driver would
refuse to run with fc disabled.

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