mmc: Remove redundant spi driver bus initialization

In ancient times it was necessary to manually initialize the bus field of an
spi_driver to spi_bus_type. These days this is done in spi_driver_register(),
so we can drop the manual assignment.

The patch was generated using the following coccinelle semantic patch:
// <smpl>
@@
identifier _driver;
@@
struct spi_driver _driver = {
	.driver = {
-		.bus = &spi_bus_type,
	},
};
// </smpl>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Chris Ball <cjb@laptop.org>