ice: Validate ring existence and its q_vector per VSI

When stopping Tx rings, we use 'i' as an ring array index for looking up
whether the ice_ring exists and have assigned a q_vector. This checks
rings only within a given TC and we need to go through every ring in
VSI. Use 'q_idx' instead.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
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>