[media] s5c73m3: 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_register_driver(), so we can drop the manual assignment.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>