ice: Sanitize ice_ena_vsi and ice_dis_vsi

1. ndo_open and ndo_stop are implemented by ice_open and ice_stop
   respectively. When enabling/disabling VSIs, just call
   ice_open/ice_stop instead of ndo_open/ndo_stop.

2. Rework logic around rtnl_lock/rtnl_unlock

3. In ice_ena_vsi, remove an unnecessary stack variable and return
   0 instead of err when __ICE_NEEDS_RESTART is not set.

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>