if_watchdog and if_timer. The driver already contained an optional stats
timer that individual attachments could use to provide a 'tick' event. The
stats timer only ran if the tick function pointer was non-NULL and the
attachment's tick routine had to call callout_reset(), etc. Now the driver
always schedules a stat timer and manages the callout_reset() internally.
This timer is used to drive the watchdog and will also call the attachment's
'tick' handler if one is provided.
Tested by: WATANABE Kazuhiro
b1f459a01b