Instead of an allocation of a single u16 per channel, allocate them all in
a single chunk which greatly reduces memory fragmentation and also the
overhead to track the allocated memory. Also put the channel data in
driver data where it's cheaper to determine the address (no function call
involved, just a trivial pointer addition).

This also allows to get rid of the request and free callbacks.

The only cost is that the channel data is allocated early, and even for
unused channels.
Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20230705080650.2353391-8-u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>