V4L/DVB (9075): gspca: Bad check of returned status in i2c_read() spca561.

This makes auto gain functional on 04fc:0561.

Signed-off-by: Shane <gnome42@gmail.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>