V4L/DVB: ir-core: centralize sysfs raw decoder enabling/disabling

With the current logic, each raw decoder needs to add a copy of the exact
same sysfs code. This is both unnecessary and also means that (re)loading
an IR driver after raw decoder modules have been loaded won't work as
expected.

This patch moves that logic into ir-raw-event and adds a single sysfs
file per device.

Reading that file returns something like:

	"rc5 [rc6] nec jvc [sony]"

(with enabled protocols in [] brackets)

Writing either "+protocol" or "-protocol" to that file will
enable or disable the according protocol decoder.

An additional benefit is that the disabling of a decoder will be
remembered across module removal/insertion so a previously
disabled decoder won't suddenly be activated again. The default
setting is to enable all decoders.

This is also necessary for the next patch which moves even more decoder
state into the central raw decoding structs.

Signed-off-by: David Härdeman <david@hardeman.nu>
Acked-by: Jarod Wilson <jarod@redhat.com>
Tested-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>