V4L/DVB (3575): Cxusb: fix i2c debug messages for bluebird devices

Only the Medion boxes return 0x08 after an i2c read/write.
The bluebird devices do not return anything at all.
This patch conditionalizes the test for the 0x08 return code
to produce a warning message when using the Medion box, only.

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