staging: wilc1000: refactor handle_set_mcast_filter()

Refactor handle_set_mcast_filter() by making use of put_unaligned32() to
pack the data instead of byte operation.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>