pcm: route: Allow chmap syntax for slave channels in ttable

Instead of writing e g "0" and "1", one can now write "FL" and "FR" instead.

E g:
	ttable.0.FL 1
	ttable.1.FR 1
	ttable.2.LFE 1

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
pcm: route: Select slave chmap based on ttable information

It means we need to initialize this order:

 1) Read the ttable to figure out which channels are present
 2) Open slave pcm and find a matching chmap
 3) Determine size of ttable (this can now depend on the chmap)
 4) Read ttable coefficients
 5) At prepare time, select the matching chmap

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
conf: Allow 2.1 surround to use different number of channels

This way, cards that support LFE on four channels (e g laptop with
internal subwoofer) can do that, and other cards on a six channel setup
can use that as well.

Well, note that there is still a reference to "pcm.surround51" left here.
In practice, for HDA Intel sound cards this does not matter as both
surround51 and surround40 reference the same definition.
(And that's the only card I currently know of that actually does
surround2.1 over four channels.)

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>