staging: wilc1000: move 'periodic_rssi' as part of 'wilc_vif' struct

Refactor code to move 'periodic_rssi' as part of wilc_vif struct. Move
'dummy_statistics' from 'wilc' struct to 'wilc_vif' to maintain for
each interface separately.

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