brcmfmac: set dongle mode accordingly when interface up

The mode of WiFi dongle should be initialized in brcmf_cfg80211_up
which get called when network interface is brought up. Otherwise
brcmf_cfg80211_get_station would return error.

Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>