rust: driver: fix broken intra-doc links to example driver types

rust: driver: fix broken intra-doc links to example driver types

The `auxiliary` and `pci` modules are conditional on
`CONFIG_AUXILIARY_BUS` and `CONFIG_PCI` respectively. When these are
disabled, the intra-doc links to `auxiliary::Driver` and `pci::Driver`
break, causing rustdoc warnings (or errors with `-D warnings`).

error: unresolved link to `kernel::auxiliary::Driver`
  --> rust/kernel/driver.rs:82:28
   |
82 | //! [`auxiliary::Driver`]: kernel::auxiliary::Driver
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `auxiliary` in module `kernel`

Fix this by making the documentation for these examples conditional on
the corresponding configuration options.

Fixes: 970a7c68788e ("driver: rust: expand documentation for driver infrastructure")
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reported-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Closes: https://lore.kernel.org/rust-for-linux/20251209.151817.744108529426448097.fujita.tomonori@gmail.com/
Link: https://patch.msgid.link/20251227-driver-types-v1-1-1916154fbe5e@google.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>