rust: phy: use `srctree`-relative links

The relative paths like the following are bothersome and don't work
with `O=` builds:

//! C headers: [`include/linux/phy.h`](../../../../../../../include/linux/phy.h).

This updates such links by using the `srctree`-relative link feature
introduced in 6.8-rc1 like:

//! C headers: [`include/linux/phy.h`](srctree/include/linux/phy.h).

Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>