wifi: ath: Use is_multicast_ether_addr() to check multicast Ether address

Use is_multicast_ether_addr() to perform the Checking.

Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230814124212.302738-2-ruanjinjie@huawei.com