net: ieee802154: 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>
Acked-by: Varka Bhadram <varkabhadram@gmail.com>
Acked-by: Alan Ott <alan@signal11.us>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>