iio: trigger: stm32-timer: fix quadrature mode get routine

Fixes: 4adec7d ("iio: stm32 trigger: Add quadrature encoder device")

SMS bitfiled is mode + 1. After reset, upon boot, SMS = 0. When reading
from sysfs, stm32_get_quadrature_mode() returns -1 (e.g. -EPERM) which is
wrong error code here. So, check SMS bitfiled matches valid encoder mode,
or return -EINVAL.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/trigger/stm32-timer-trigger.c

@@ -571,11 +571,14 @@ static int stm32_get_quadrature_mode(struct iio_dev *indio_dev,
 {
 	struct stm32_timer_trigger *priv = iio_priv(indio_dev);
 	u32 smcr;
+	int mode;
 
 	regmap_read(priv->regmap, TIM_SMCR, &smcr);
-	smcr &= TIM_SMCR_SMS;
+	mode = (smcr & TIM_SMCR_SMS) - 1;
+	if ((mode < 0) || (mode > ARRAY_SIZE(stm32_quadrature_modes)))
+		return -EINVAL;
 
-	return smcr - 1;
+	return mode;
 }
 
 static const struct iio_enum stm32_quadrature_mode_enum = {