[media] gspca_sonixb: Properly wait between i2c writes

We must wait for the previous i2c write to complete before starting a new
one. Sofar we were getting away with this, but it seems that some parts
of the usb-subsystem has been sped up making us go to fast :)
This fixes streaming on sn9c103 based cams not working with an
"i2c_w error" error.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>