ALSA: Echoaudio - Add firmware cache #1

Changes the way the firmware is passed through functions.

When CONFIG_PM is enabled the firmware cannot be released because the
driver will need it again to resume the card. 
With this patch the firmware is passed as an index of the struct
firmware card_fw[] in place of a pointer. That same index is then used
to locate the firmware in the firmware cache.

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