rust: dma: fix broken intra-doc links

The `pci` module is conditional on CONFIG_PCI. When it's disabled, the
intra-doc link to `pci::Device` causes rustdoc warnings:

warning: unresolved link to `::kernel::pci::Device`
  --> rust/kernel/dma.rs:30:70
   |
30 | /// where the underlying bus is DMA capable, such as [`pci::Device`](::kernel::pci::Device) or
   |                                                                      ^^^^^^^^^^^^^^^^^^^^^ no item named `pci` in module `kernel`

Fix this by making the documentation conditional on CONFIG_PCI.

Fixes: d06d5f6 ("rust: dma: implement `dma::Device` trait")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://patch.msgid.link/20251231045728.1912024-1-fujita.tomonori@gmail.com
[ Keep the "such as" part indicating a list of examples; fix typos in
  commit message. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: fujita

rust/kernel/dma.rs

@@ -27,8 +27,9 @@ pub type DmaAddress = bindings::dma_addr_t;
 /// Trait to be implemented by DMA capable bus devices.
 ///
 /// The [`dma::Device`](Device) trait should be implemented by bus specific device representations,
-/// where the underlying bus is DMA capable, such as [`pci::Device`](::kernel::pci::Device) or
-/// [`platform::Device`](::kernel::platform::Device).
+/// where the underlying bus is DMA capable, such as:
+#[cfg_attr(CONFIG_PCI, doc = "* [`pci::Device`](kernel::pci::Device)")]
+/// * [`platform::Device`](::kernel::platform::Device)
 pub trait Device: AsRef<device::Device<Core>> {
     /// Set up the device's DMA streaming addressing capabilities.
     ///