Remember the active infoframe, so as to avoid stop/restart infoframe
transmission when switching between audio clips of the same format.

Proposed by Shang and David.

CC: Shane W <shane-alsa@csy.ca>
CC: David Härdeman <david@hardeman.nu>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>