staging: wilc1000: avoid use of static variable 'rssi'

Instead of static variable now allocating the data and passing to
handle_get_rssi() to fill the rssi information.

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