staging: wilc1000: refactor code to avoid use of wilc_set_multicast_list global

Refactor the code to avoid use of 'wilc_set_multicast_list' global
variable. Allocate the memory required to keep the multicast mac address
and pass it to the worker thread.

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