[ALSA] echoaudio, add TLV support

This patch adds TLV support to the echoaudio driver.
All gains are in the range -127dB to +6dB with steps of 1dB, and -128 is
mute. VU-meters levels go from -128 to 0dB. The input gain of the Layla20
ranges from -25dB to +25dB in steps of 0.5dB.

Signed-off-by: Giuliano Pochini <pochini@shiny.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>