The unpack functions just read from the passed in buffer,
so make it const.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: linux-media@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180920185145.1912-2-ville.syrjala@linux.intel.comAcked-by: Hans Verkuil <hans.verkuil@cisco.com>