iio: frequency: admv4420: use dev_err_probe

iio: frequency: admv4420: use dev_err_probe

Use dev_err_probe() instead of dev_err() in the probe path to ensure
proper handling of deferred probing and to simplify error handling.

Also fix the format specifier for vco_freq_hz from %lld to %llu since
it is u64 (unsigned), and add missing newline to the error message.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>