media: allegro: Fix use after free on error

The "channel" is added to the "dev->channels" but then if
v4l2_m2m_ctx_init() fails then we free "channel" but it's still on the
list so it could lead to a use after free.  Let's not add it to the
list until after v4l2_m2m_ctx_init() succeeds.

Fixes: cc62c747 ("media: allegro: add missed checks in allegro_open()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>