sfc: Use 'skb_add_rx_frag()' instead of hand coding it

Some lines of code can be merged into an equivalent 'skb_add_rx_frag()'
call which is less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>