rust: pci: implement Driver::unbind()

rust: pci: implement Driver::unbind()

Currently, there's really only one core callback for drivers, which is
probe().

Now, this isn't entirely true, since there is also the drop() callback of
the driver type (serving as the driver's private data), which is returned
by probe() and is dropped in remove().

On the C side remove() mainly serves two purposes:

  (1) Tear down the device that is operated by the driver, e.g. call bus
      specific functions, write I/O memory to reset the device, etc.

  (2) Free the resources that have been allocated by a driver for a
      specific device.

The drop() callback mentioned above is intended to cover (2) as the Rust
idiomatic way.

However, it is partially insufficient and inefficient to cover (1)
properly, since drop() can't be called with additional arguments, such as
the reference to the corresponding device that has the correct device
context, i.e. the Core device context.

This makes it inefficient (but not impossible) to access device
resources, e.g. to write device registers, and impossible to call device
methods, which are only accessible under the Core device context.

In order to solve this, add an additional callback for (1), which we
call unbind().

The reason for calling it unbind() is that, unlike remove(), it is *only*
meant to be used to perform teardown operations on the device (1), but
*not* to release resources (2).

Link: https://lore.kernel.org/r/20250621195118.124245-8-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>