i40e: remove redundant assignment

Remove a redundant assignment of the software ring pointer in the i40e
driver. The variable is assigned twice with no use in between, so just
get rid of the first occurrence.

Fixes: 3b4f0b66 ("i40e, xsk: Migrate to new MEM_TYPE_XSK_BUFF_POOL")
Signed-off-by: Marek Majtyka <marekx.majtyka@intel.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: George Kuruvinakunnel <george.kuruvinakunnel@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>