pwm: Add missing "CONFIG_" prefix

The IS_ENABLED() use was missing the CONFIG_ prefix which would have
lead to skipping this code.

Fixes: 3ad1f3a3 ("pwm: Implement some checks for lowlevel drivers")
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>