V4L/DVB (8667): gspca: Bad probe of Z-Star/Vimicro webcams with pas106 sensor.

This patch moves the detection of the 2wr SIF pas106b sensor to before
detecting other sensors.
Patch change by jfm: Check SIF for identified webcams only.

Signed-off-by: Hans de Goede <j.w.r.degoede@hhs.nl>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>