regmap: sdw-mbq: don't call an unset readable_reg callback

regmap: sdw-mbq: don't call an unset readable_reg callback

regmap_sdw_mbq_poll_busy() decides whether to poll the Function Busy bit
by calling ctx->readable_reg(), which is a straight copy of
config->readable_reg. That callback is optional: regmap_readable() treats
a NULL ->readable_reg as "every register is readable", and drivers rely on
that. es9356 and tac5xx2-sdw both build an MBQ regmap without one.

Since commit ca1b11b36d82 ("regmap: sdw-mbq: Allow defers on undeferrable
controls") the poll runs on every -ENODATA, not only for Controls the
driver marked deferrable, so any of those devices answering
COMMAND_IGNORED takes the kernel through a NULL function pointer.

Treat a missing callback the way the rest of regmap does and poll.

Fixes: 5bc493bf0c37 ("regmap: sdw-mbq: Add support for SDCA deferred controls")
Signed-off-by: Andrey Golovko <andrey.golovko@gmail.com>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://patch.msgid.link/20260811184500.5312-1-andrey.golovko@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>