iio: adc: ad9081: Validate sysfs_emit() result in ext_info_read()

Ensure the return value of sysfs_emit() is properly checked in
ad9081_ext_info_read() to catch cases where no data is written or
an error occurs. This prevents misleading success returns when the
buffer emission fails.
Set ret = -EINVAL if sysfs_emit() returns zero or a negative value,
aligning with expected error handling practices in sysfs attribute reads.

Signed-off-by: Michael Hennerich <[email protected]>
(cherry picked from commit ba297cc)

Author: mhennerich

drivers/iio/adc/ad9081.c

@@ -1334,6 +1334,8 @@ static ssize_t ad9081_ext_info_read(struct iio_dev *indio_dev,
 		}
 		ret = sysfs_emit(buf, "%lld\n",
 			phy->tx_ffh_hopf_vals[phy->tx_ffh_hopf_index[cddc_num]][cddc_num]);
+		if (ret <= 0)
+			ret = -EINVAL;
 		break;
 	default:
 		ret = -EINVAL;