ixgbevf: Indicate removal state explicitly

Add a bit, __IXGBEVF_REMOVING, to indicate that the module is being
removed. The __IXGBEVF_DOWN bit had been overloaded for this purpose,
but that leads to trouble. A few places now check both __IXGBEVF_DOWN
and __IXGBEVF_REMOVING.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>