ixgbevf: Protect ixgbevf_reset_subtask from remove event

In ixgbevf_reset_subtask We weren't verifying that the port haven't
been removed, we are with this patch.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>