ixgbe: cleanup unclear references to reg_idx

ixgbe: cleanup unclear references to reg_idx

There are a number of places where we use the variable j to contain the
register index of the ring.  Instead of using such a non-descriptive
variable name it is better that we name it reg_idx so that it is clear what
the variable contains.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>