staging: rtl8188eu: use ETH_ALEN

Use ETH_ALEN instead of hard coded array size.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Link: https://lore.kernel.org/r/20200929062847.23985-4-straube.linux@gmail.com


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