iio:accel:stk8312: check for invalid value

Revision 1.2 of the datasheet recommends on page 22 to only write non-zero
values read from OTP register 0x70 into AFECTRL register.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/accel/stk8312.c b/drivers/iio/accel/stk8312.c
index a553736..8280a37 100644
--- a/drivers/iio/accel/stk8312.c
+++ b/drivers/iio/accel/stk8312.c
@@ -150,6 +150,8 @@
 		goto exit_err;
 
 	ret = i2c_smbus_read_byte_data(client, STK8312_REG_OTPDATA);
+	if (ret == 0)
+		ret = -EINVAL;
 	if (ret < 0)
 		goto exit_err;