ixgbe: Indicate removal state explicitly

Add a bit, __IXGBE_REMOVING, to indicate that the module is being
removed. The __IXGBE_DOWN bit had been overloaded for this purpose,
but that leads to trouble. A few places now check both __IXGBE_DOWN
and __IXGBE_REMOVE. Notably, setting either bit will prevent service
task execution.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>