media: vimc: sen: register subdevice only after initialization

vimc_sen_add function first registers the subdevice and then
calls tpg_alloc. If tpg_alloc fails it unregisters the subdevice
and then frees vsen, this cause double free since the release
callback that follows subdevice unregistration also frees vsen.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Acked-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>