fsi: Assign driver's bus in fsi_driver_register()

fsi: Assign driver's bus in fsi_driver_register()

Instead of letting each driver assign the bus, do it once in the fsi
driver register function.

Simplify the fsi drivers that are living in drivers/fsi accordingly.

Once all fsi drivers dropped assigning the bus, fsi_bus_type can be made
a static variable.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Acked-by: Eddie James <eajames@linux.ibm.com>
Link: https://patch.msgid.link/54804c2cd4d84a6b5fb679831122b6acdd36b168.1765279318.git.u.kleine-koenig@baylibre.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>