staging: rtl8712: Use ETH_ALEN instead of hardcoded value

Use macro ETH_ALEN which defines the number of octets in
an ethernet address.

Signed-off-by: Mauro Dreissig <mukadr@gmail.com>
Link: https://lore.kernel.org/r/20200705143552.9368-4-mukadr@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>