ALSA: AC97: add full duplex support for atmel AT91 and AVR.

This patch add full duplex support on AT91 and AVR.
It was a bug: we needed to check first if there are some chips opened so we
could enable both reception and sending of the data.

Signed-off-by: Sedji Gaouaou <sedji.gaouaou@atmel.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>