V4L/DVB: saa7134: Add support for both positive and negative edge IRQ

The code that enables IRQ for the Remote Controller on saa7134 is a little
messy: it is outside saa7134-input, it checks if RC is GPIO based, and
it mixes both serial raw decode with parallel reads from a hardware-based
IR decoder.

Also, currently, it doesn't allow to trigger both transition edges at GPIO16
and GPIO18 lines. A rework on the code is needed to provide a better way
to specify what saa7134-input needs, maybe even moving part of the code from
saa7134-core and saa7134-cards into saa7134-input.

Yet, as a large rework is happening at RC core, it is better to wait until
the core changes stablize, in order to rework saa7134 RC internals.While
this don't happen, let's just change the logic a little bit to allow
enabling IRQ to be generated on both edge transitions, in order to better
support pulse/space raw decoders.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>