sfc: Use canonical pointer type for MAC address in efx_set_mac_address()

Functions such as is_valid_ether_addr() expect u8 *, so use that
instead of char *.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>