ice: use ice_fill_eth_hdr() in ice_fill_sw_rule()

Use the already existing helper function to fill Ethernet header. Also
replace sizeof with a (also existing) macro to reduce the number of
variables.

Suggested-by: Martyna Szapar-Mudlaw <martyna.szapar-mudlaw@intel.com>
Signed-off-by: Marcin Szycik <marcin.szycik@intel.com>
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>