i40e: fix warning about shadowed ring parameter

In commit 147e81ec

 ("i40e: Test memory before ethtool alloc succeeds")
code was added to handle ring allocation on systems with low memory.

It shadowed the ring parameter pointer by introducing a local ring
pointer inside the for loop. Most of the code in the loop already just
accessed the ring via &rx_rings[i]. Since most of the code already does
this, just remove the local variable.

If someone considers it worth keeping a local around, they should use it
for the whole section instead of just a couple of accesses.

This fixes a warning when -Wshadow is enabled

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>