staging:iio:ad7606: Avoid allocating buffer for each data capture

Currently the ad7606 driver dynamically allocates and frees a transfer
buffer each time a sample capture is performed in buffered mode, which
introduces unnecessary overhead. The driver state struct already contains a
buffer that is used for transfers in one-shot mode. This buffer is large
enough to hold all samples, but not the timestamp that might be present in
buffered mode. Extend the buffer size to be able to contain the timestamp
and update the buffered capture function to use this buffer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>