UBUNTU-CVE-2026-74738: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: regmap: sdw-mbq: don't call an unset readablereg callback regmapsdwmbqpollbusy() decides whether to poll the Function Busy bit by calling ctx->readablereg(), which is a straight copy of config->readablereg. That callback is optional: regmapreadable() treats a NULL ->readablereg 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 COMMANDIGNORED takes the kernel through a NULL function pointer. Treat a missing callback the way the rest of regmap does and poll.