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.