CVE-2022-48927 iio: adc: tsc2046: fix memory corruption by preventing array overflow

In the Linux kernel, the following vulnerability has been resolved:

iio: adc: tsc2046: fix memory corruption by preventing array overflow

On one side we have indio_dev->num_channels includes all physical channels +
timestamp channel. On other side we have an array allocated only for
physical channels. So, fix memory corruption by ARRAY_SIZE() instead of
num_channels variable.

Note the first case is a cleanup rather than a fix as the software
timestamp channel bit in active_scanmask is never set by the IIO core.