ixgb: fix flow control

Make default flow control only have *sending* of flow control packets
enabled, and fix to disable / enable flow control correctly. Set flow
control defaults to disable receiving flow control from the link
partner, to fix the transmit fifo overlow errata

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: John Ronciak <john.ronciak@intel.com>