ixgbe: fix several counter register errata

Several counters behave differently on 82598 causing them to display
incorrect values. Adjust the accounting so the reported numbers
make sense and do not double count or represent the wrong item.

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