SPI: Remove sound card self registration code from the TSC2301 mixer

Feature to create and register sound card instance in tsc2301-mixer.c
was used in early development of the driver but otherwise there is no use
for it so patch removes that unused code.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>