ixgbe: Add iterator for cycling through rings on a q_vector

Since there are multiple spots where we have to cycle through all of the
rings on a q_vector it makes sense to just add a function for iterating
through all of them.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>