V4L/DVB (5449): M920x: add error messages for debugging purposes

Signed-off-by: Pierre Willenbrock <pierre@pirsoft.dnsalias.org>
Signed-off-by: Aapo Tahkola <aet@rasterburn.org>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>