staging: most: sound: create one sound card w/ multiple PCM devices per MOST device

This patch avoids that a sound card is created and registered with ALSA
every time a channel is being linked. Instead the channels are hooked on
the same card, which is registered not until the final link has been added
to the component. The string provided by user space that used to be the
card name becomes the PCM device name. The user space API to add a link is
being expanded by a "create" flag to trigger the registration.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>