staging: most: sound: rename variable

Since the channels of a MOST device are now being represented as
individual PCM devices of one sound card, the variable card_name is not
suitable anymore to describe them. Therefore, this patch renames the
variable to device_name.

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>