CVE-2026-31487: spi: use generic driver_override infrastructure

In the Linux kernel, the following vulnerability has been resolved:

spi: use generic driveroverride infrastructure

When a driver is probed through driverattach(), the bus' match() callback is called without the device lock held, thus accessing the driveroverride field without a lock, which can cause a UAF.

Fix this by using the driver-core driveroverride infrastructure taking care of proper locking internally.

Note that calling match() from driverattach() without the device lock held is intentional. [1]

Also note that we do not enable the driveroverride feature of struct bustype, as SPI - in contrast to most other buses - passes "" to sysfsemit() when the driveroverride pointer is NULL. Thus, printing "\n" instead of "(null)\n".