s390/qeth: avoid using is_multicast_ether_addr_64bits on (u8 *)[6]
*ether_addr*_64bits functions have been introduced to optimize
performance critical paths, which access 6-byte ethernet address as u64
value to get "nice" assembly. A harmless hack works nicely on ethernet
addresses shoved into a structure or a larger buffer, until busted by
Kasan on smth like plain (u8 *)[6].
qeth_l2_set_mac_address calls qeth_l2_remove_mac passing
u8 old_addr[ETH_ALEN] as an argument.
Adding/removing macs for an ethernet adapter is not that performance
critical. Moreover is_multicast_ether_addr_64bits itself on s390 is not
faster than is_multicast_ether_addr: