iio: adc: vf610_adc: add helper function to read samples

This is a precursor change to make it simpler to remove the 'mlock'
usage. Having the code in it's own helper function, also makes it easier
to read the error paths.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>
Link: https://lore.kernel.org/r/20221004134909.1692021-10-nuno.sa@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>