[WATCHDOG] iTCO_wdt : correct status clearing

The iTCO_wdt code was not clearing the correct bits.
It now clears the timeout status bit and then the
SECOND_TO_STS bit and then the BOOT_STS bit.
Note: we should first clear the SECOND_TO_STS bit
before clearing the BOOT_STS bit.

Signed-off-by: Wim Van Sebroeck <wim@iguana.be>