watchdog: sp5100_tco: Fix watchdog disable bit

According to all published information, the watchdog disable bit for SB800
compatible controllers is bit 1 of PM register 0x48, not bit 2. For the
most part that doesn't matter in practice, since the bit has to be cleared
to enable watchdog address decoding, which is the default setting, but it
still needs to be fixed.

Cc: Zoltán Böszörményi <zboszor@pr.hu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>