staging: brcm80211: remove typedef for struct wl_timer

Typedefinitions for structures are considered affecting the
code readability. This removes use of wl_timer_t which is
typedef for struct wl_timer.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Brett Rudley <brudley@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>