brcm80211: smac: set interface down on reset

This change marks interface as down on reset, otherwise the driver can't
reinitialize itself properly.

Without the change a transient problem turns out to be critical and leads
to inavailability to reset the driver without brcmsmac module unload/load
cycle:

    ieee80211 phy0: wl0: PSM microcode watchdog fired at 5993 (seconds). Resetting.
    brcms_c_dpc : PSM Watchdog, chipid 0xa8d9, chiprev 0x1
    ieee80211 phy0: wl0: fatal error, reinitializing
    ieee80211 phy0: Hardware restart was requested
    ieee80211 phy0: brcms_ops_start: brcms_up() returned -19

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Cc: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>