iio: adc: max1027: unlock on error path in max1027_read_single_value()

If max1027_wait_eoc() fails then call iio_device_release_direct_mode()
before returning.

Fixes: a0e83165

 ("iio: adc: max1027: Introduce an end of conversion helper")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/YsbztVuAXnau2cIZ@kili
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>