V4L/DVB (12351): gspca - sn9c20x: Misc fixes

* use i2c_w instead of reg_w
* return error on failure
* read the correct number of bytes

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