staging: mt29f_spinand: 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: devel@driverdev.osuosl.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>