devm_ioremap() doesn't return error pointers, it returns NULL on error.
Update the check accordingly.

Fixes: e86bd43b ("watchdog: sa1100: use platform device registration")
Signed-off-by: Chen Ni <nichen@iscas.ac.cn>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240426075808.1582678-1-nichen@iscas.ac.cnSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>