brcmsmac: improve readability on addresses copy

A static analyzer identified as a potential bug the copy of
12 bytes from a 6 bytes array to a 6 bytes array. Both
arrays are 6 bytes addresses.

Although not being a real bug, it is not immediately clear
why is done this way: next 6 bytes address, contiguous to
the first one, must also be copied to next contiguous 6 bytes
address of the destination.

Copying each one separately will make both static analyzers
and reviewers happier.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210511070257.7843-1-ihuguet@redhat.com