media: hantro: streamline open, reuse error path

This adds a label to the error path to avoid calling
v4l2_m2m_ctx_init() and kfree(ctx) in two places each.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>