[media] rtl28xxu: fix buffer overflow when probing Rafael Micro r820t tuner

As suggested by Antti, this patch replaces:
https://patchwork.kernel.org/patch/2649861/


The buffer overflow is fixed by reading only the r820t ID register.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Acked-by: Antti Palosaari <crope@iki.fi>
Reviewed-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>