rust: phy: implement `Send` for `Registration`

In preparation for requiring `Send` for `Module` implementations in the
next patch.

Cc: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: Trevor Gross <tmgross@umich.edu>
Cc: netdev@vger.kernel.org
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240328195457.225001-2-wedsonaf@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>