staging: wilc1000: refactor wilc_wlan_set_bssid()

Refactor code by making use of eth_zero_addr() to clear the mac address
value in wilc_wlan_set_bssid().

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