brcmfmac: fix get rssi by clearing getvar struct.

The function brcmf_cfg80211_get_station requests the RSSI from
the device. The complete structure used needs to be cleared
before sending the request to firmware. Otherwise the request
fails filling the logs with "Could not get rssi (-2)" messages.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>