watchdog: remove error message when unable to allocate watchdog device

The dev_err message is superfluous because the failure is already
printed by dev_kzalloc, so remove it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>