brcm80211: smac: decreased timer callback irq level

Timer functions were called at soft-irq level, leading to the limitation
that mutexes could not be used. Lifted this limitation by migrating to
work queues.

Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>