watchdog: pass USEC_INFINITY to watchdog_setup() to reuse the programmed timeout...

watchdog: pass USEC_INFINITY to watchdog_setup() to reuse the programmed timeout value

This patch changes the meaning of USEC_INFINITY value for the watchdog
module.

Previously passing this value was a NOP. It now has a special meaning: it
requests the watchdog module to read the programmed timeout value and reuse it
for pinging the device.

This is mostly useful when the watchdog is started by the firmware and there's
no way to reconfigure the timeout with a different value
afterwards. "RuntimeWatchdogSec=infinity" in system.conf can now be used rather
than putting an arbitrary value that PID1 will fail to set (even if it still
felt back to the programmed timeout).

Please note that "infinity" is not supposed to restore the default value of the
firmware. If the value is changed after booting then "infinity" would simply
reuse the current programmed value. IOW it's a NOP unless the watchdog was
previously closed and in that case it will be reopened and the last programmed
value reused.