[WATCHDOG] iTCO_wdt: fix memory corruption when RCBA is disabled by hardware

According to 9.1.33 on p.343 of ICH8.pdf RCBA can be disabled by
hardware if bit 0 of RCBA register is not set.

Perform correct check for this to prevent memory corruption under
some virtual machines where this feature is disabled.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Vasily Averin <vvs@openvz.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>