e1000: workaround for the ESB2 NIC RX unit issue

In rare occasions, ESB2 systems would end up started without the RX
unit being turned on. Add a check that runs post-init to work around
this issue.

Originally from Jesse Brandeburg <jesse.brandeburg@intel.com>,
rewritten to use feature flags by me.

Signed-off-by: Jeff Garzik <jeff@garzik.org>