ALSA: pci/echoaudio: remove 'set but not used' warning

ALSA: pci/echoaudio: remove 'set but not used' warning

Fix W=1 warning. One variable is only used in a conditionally-compiled
block, mark as __maybe_unused

sound/pci/echoaudio/echoaudio.c: In function ‘snd_echo_probe’:
sound/pci/echoaudio/echoaudio.c:1958:6: warning: variable ‘i’ set but
not used [-Wunused-but-set-variable]
 1958 |  int i, err;
      |      ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-7-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>