IIO: Documentation: iio_utils: Prevent buffer overflow

The first part of build_channel_array()identifies the number of enabled channels.
Further down this count is used to allocate the ci_array. The next section parses the
scan_elements directory again, and fills ci_array regardless if the channel is enabled or not.
So if less than available channels are enabled ci_array memory is overflowed.

This fix makes sure that we allocate enough memory. But the whole approach looks a bit
cumbersome to me. Why not allocate memory for MAX_CHANNLES, less say 64
(I never seen a part with more than that channels). And skip the first part entirely.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>