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: NHartmut Knaack <knaack.h@gmx.de>
Reviewed-by: NTiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>

drivers/iio/accel/stk8312.c

@@ -150,6 +150,8 @@ static int stk8312_otp_init(struct stk8312_data *data)
 		goto exit_err;
 
 	ret = i2c_smbus_read_byte_data(client, STK8312_REG_OTPDATA);
+	if (ret == 0)
+		ret = -EINVAL;
 	if (ret < 0)
 		goto exit_err;