media: serial_ir: change "ignoring spike" to debug level

At least on my system with a self-made IR receiver, my kernel log is filled
with:

serial_ir serial_ir.0: ignoring spike: 1 1 1419988034627194ns 1419956080709377ns

These messages happen at random and do not prevent the receiver from
working. Also I cannot change the features of the IC, therefore they are not
useful. Probably they are not useful at all.

However they fill the console, they accumulate and fill the dmesg log, by
doing this, they prevent me from seeing important message.

Signed-off-by: Bodo Eggert <7eggert@gmx.de>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>