iio: ssp_sensors: 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>
To: Jonathan Cameron <jic23@kernel.org>
To: Lars-Peter Clausen <lars@metafoo.de>
To: Karol Wrona <k.wrona@samsung.com>
To: linux-iio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Acked-by: Karol Wrona <k.wrona@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>