Currently USB-audio driver manages the auto-pm of the primary
interface although a card may consist of multiple interfaces.
This may leave the secondary and other interfaces left running
unnecessarily after the auto-suspend.

This patch allows the driver managing the auto-pm of all bundled
interfaces per card.  The chip->pm_intf field is extended as
chip->intf[] to contain the array of assigned interfaces, and the
runtime-PM is performed to all those interfaces.
Tested-by: Macpaul Lin <macpaul.lin@mediatek.com>
Link: https://lore.kernel.org/r/20200605064117.28504-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>