watchdog: sp5100_tco: Use standard error codes

By using standard error codes, we can identify and return more than one
error condition.

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