Makes the IPMI watchdog more consistent with the other watchdog drivers.

I have tested this and it seems to work correctly.  I also added docs for
the interface change.

- support disabling watchdog by writing 'V' to device.
- unify printk()
- use atomic bit operations on ipmi_wdog_open
Signed-off-by: Arkadiusz Miskiewicz <arekm@pld-linux.org>
Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>