staging: r8188eu: use helper to check for broadcast address

Use the is_broadcast_ether_addr function to check for a
broadcast address.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211020195401.12931-3-martin@kaiser.cx


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