staging: r8188eu: use standard multicast addr check

Use is_multicast_ether_addr to check for a multicast address instead of
reimplementing this check in the driver.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20221023170808.46233-2-martin@kaiser.cx


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