staging: brcm80211: fix thread blocking issue in brcmf_sdbrcm_bus_stop()

The function brcmf_sdbrcm_bus_stop() terminates the watchdog and dpc
thread, but this function can be called from the dpc thread itself.
Stopping the dpc thread is only done when it is not the 'current'
thread.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>