ALSA: echoaudio: simplify get_audio_levels

The loop optimizer seems to go astray here, and produces some warnings
that don't seem valid.

Still, the code can be simplified -- just clear the whole array at the
beginning, and fill in whatever values are valid on the platform.

Warnings before this change (GCC 8.2.0 ARM allmodconfig):

In file included from ../sound/pci/echoaudio/gina24.c:115:
../sound/pci/echoaudio/echoaudio.c: In function 'snd_echo_vumeters_get':
../sound/pci/echoaudio/echoaudio_dsp.c:647:9: warning: iteration 1073741824 invokes undefined behavior [-Waggressive-loop-optimizations]
In file included from ../sound/pci/echoaudio/layla24.c:112:
../sound/pci/echoaudio/echoaudio.c: In function 'snd_echo_vumeters_get':
../sound/pci/echoaudio/echoaudio_dsp.c:658:9: warning: iteration 1073741824 invokes undefined behavior [-Waggressive-loop-optimizations]
../sound/pci/echoaudio/echoaudio_dsp.c:647:9: warning: iteration 1073741824 invokes undefined behavior [-Waggressive-loop-optimizations]

Signed-off-by: Olof Johansson <olof@lixom.net>
Link: https://lore.kernel.org/r/20191207224953.25944-1-olof@lixom.net
Signed-off-by: Takashi Iwai <tiwai@suse.de>