soc: mediatek: mtk-cmdq: Move cmdq_instruction init to declaration

soc: mediatek: mtk-cmdq: Move cmdq_instruction init to declaration

Move, where possible, the initialization of struct cmdq_instruction
variables to their declaration to compress the code.

While at it, also change an instance of open-coded mask to use the
GENMASK() macro instead, and instances of `ret = func(); return ret;`
to the equivalent (but shorter) `return func()`.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>