fm10k: reduce scope of the ring variable

Reduce the scope of the ring local variable in the fm10k_assign_l2_accel
function.

This was detected by cppcheck and resolves the following warning
produced by that tool:

[fm10k_netdev.c:1447]: (style) The scope of the variable 'ring' can be
reduced.

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>