[media] tda18271: properly report read errors in tda18271_get_id

Until now, if there is a read error in tda18271_get_id, the driver
reports "Unknown device..."  Instead, check the return value of
tda18271_read_regs and display the appropriate error message.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>