ASoC: Remove unneeded use of spi_bus_type

No need to explicitly set the bus type, spi_register_driver does
that for us.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>