pmdomain: mediatek: mfg: initialize prev_o in mtk_mfg_attach_dev()

In the Linux kernel, the following vulnerability has been resolved: pmdomain: mediatek: mfg: initialize prev_o in mtk_mfg_attach_dev() mtk_mfg_attach_dev() reads prev_o on the first iteration of its loop, in "if (prev_o && prev_o->freq == o->freq)", before prev_o is assigned at the end of the loop body. On that first iteration, evaluating prev_o reads an indeterminate value. If it is non-NULL, the condition dereferences a stale or invalid pointer, potentially faulting or incorrectly skipping the first OPP. Initialize prev_o to NULL. This matches the intent as well: there is no previous OPP to compare against on the first iteration. Found with Clang's -Wconditional-uninitialized.