CVE-2026-74738 regmap: sdw-mbq: don't call an unset readable_reg callback

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:...