media: mtk-jpeg: Remove some unnecessary variables

remove is_jpgenc_multihw, is_jpgdec_multihw,
and unnecessary initialization.

Signed-off-by: kyrie wu <kyrie.wu@mediatek.com>
Signed-off-by: irui wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>