sfc: avoid duplicate error handling code in 'efx_ef10_sriov_set_vf_mac()'

'eth_zero_addr()' is already called in the error handling path. This is
harmless, but there is no point in calling it twice, so remove one.

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