media: subdev: Fix use of sd->enabled_streams in call_s_stream()

call_s_stream() uses sd->enabled_streams to track whether streaming has
already been enabled. However,
v4l2_subdev_enable/disable_streams_fallback(), which was the original
user of this field, already uses it, and
v4l2_subdev_enable/disable_streams_fallback() will call call_s_stream().

This leads to a conflict as both functions set the field. Afaics, both
functions set the field to the same value, so it won't cause a runtime
bug, but it's still wrong and if we, e.g., change how
v4l2_subdev_enable/disable_streams_fallback() operates we might easily
cause bugs.

Fix this by adding a new field, 's_stream_enabled', for
call_s_stream().

Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>