ALSA: snd-atmel-ac97c: enable interrupts to catch events for error reporting

This patch will enable interrupts from AC97C and report about error
conditions that occurs.

On channel A both overrun and underrun will be enabled depending if
playback and/or capture are enabled. On the control channel the overrun
interrupt is enabled.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>