fm10k: use a local variable for the frag pointer

In the function fm10k_xmit_frame_ring, we recently switched to using
the skb_frag_size accessor instead of directly using the size member of
the skb fragment.

This made the for loop slightly harder to read because it created a very
long line that is difficult to split up. Avoid this by using a local
variable in the for loop, so that we do not have to break the line on an
open parenthesis.

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>