iio: adc: ad7124: change setup reg allocation strategy

iio: adc: ad7124: change setup reg allocation strategy

Change the allocation strategy of the 8 SETUP registers from a least-
recently-used (LRU) to a first-come-first-served basis.

The AD7124 chips can have up to 16 channels enabled at a time in the
sequencer for buffered reads, but only have 8 SETUP configurations
(namely the OFFSET, GAIN, CONFIG and FILTER registers) that must be
shared among the 16 channels.  This means some of the channels must use
the exact same configuration parameters so that they can share a single
SETUP group of registers.  The previous LRU strategy did not keep track
of how many different configurations were requested at the same time,
so if there were more than 8 different configurations requested, some
channels would end up using the incorrect configuration because the slot
assigned to them would also be assigned to a different configuration
that wrote over it later.

Adding such tracking to solve this would make an already complex
algorithm even more complex.  Instead we can replace it with a simpler
first-come-first-serve strategy.  This makes it easy to track how many
different configurations are being requested at the same time.  This
comes at the expense of slightly longer setup times for buffered reads
since all setup registers must be written each time when a buffered read
is enabled.  But this is generally not considered a hot path where
performance is critical, so should be acceptable.

This new strategy also makes hardware debugging easier since SETUPs are
now assigned in a deterministic manner and in a logical order.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>