media: s5p-mfc: Clear workbit to handle error condition

During error on CLOSE_INSTANCE command, ctx_work_bits was not getting
cleared. During consequent mfc execution NULL pointer dereferencing of
this context led to kernel panic. This patch fixes this issue by making
sure to clear ctx_work_bits always.

Fixes: 818cd91a

 ("[media] s5p-mfc: Extract open/close MFC instance commands")
Cc: stable@vger.kernel.org
Cc: linux-fsd@tesla.com
Signed-off-by: Smitha T Murthy <smitha.t@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>