ALSA: echoaudio: add reference of struct echoaudio

added reference of struct echoaudio to free_firmware function.
this structure will be later used to get a reference of the card
when converting snd_printk to dev_* in the next patch of the series.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>