drivers: adc: mcux_adc16: Fix buffer size check for multi-channel reads

The mcux_adc16 driver's read function fails to validate the user-provided
buffer size when `adc_sequence.options` are used (extra samplings).

The calculation erroneously considered only the size required for one
channel, neglecting the total number of channels in the sequence.

This leads to a **buffer overflow** when reading multiple channels.

Fix this by using `POPCOUNT(sequence->channels)` to determine the
correct channel count and ensure the buffer size is sufficient.

Signed-off-by: Badr Bacem KAABIA <badrbacemkaabia@gmail.com>