brcmsmac: Initialize power in brcms_c_stf_ss_algo_channel_get()

wlc_phy_txpower_get_current() does a logical OR of power->flags, which
presumes that power.flags was initiliazed earlier by the caller,
unfortunately, this is not the case, so make sure we zero out the struct
tx_power before calling into wlc_phy_txpower_get_current().

Reported-by: coverity (CID 146011)
Fixes: 5b435de0 ("net: wireless: add brcm80211 drivers")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>