staging: rtl8712: prepare for const netdev->dev_addr

netdev->dev_addr will be const soon, make sure the qualifier
is respected by drivers.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20211019171243.1412240-6-kuba@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>