staging: wilc1000: refactor get_bssid() function

Refactor get_bssid() by making use of 'ieee80211_mgmt' struct. Instead
of passing the memory offset now using structure element to fetch the
bssid information.
Returning the pointer to bssid from get_bssid() instead of filing the
input argument.

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