staging: r8188eu: use helper to set broadcast address

The eth_broadcast_addr helper assigns the broadcast address to an address
array. Call this function instead of copying the address bytes manually.

Acked-by: Phillip Potter <phil@philpotter.co.uk>
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211020195401.12931-4-martin@kaiser.cx


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