e1000: consolidate managability enabling/disabling

Several bugs existed in how we handle manageability issues all
over the driver.  This patch consolidates all the managability
release and init code in two single functions and call them from
appropriate locations. This fixes several BMC packet redirect issues
and powerup/down hiccups.

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

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