rust: io: rename `io::Io` accessors

rust: io: rename `io::Io` accessors

Rename the I/O accessors provided by `Io` to encode the type as
number instead of letter. This is in preparation for Port I/O support
to use a trait for generic accessors.

Add a `c_fn` argument to the accessor generation macro to translate
between rust and C names.

Suggested-by: Danilo Krummrich <dakr@kernel.org>
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/PIO.20support/near/499460541
Signed-off-by: Fiona Behrens <me@kloenk.dev>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Acked-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250217-io-generic-rename-v1-1-06d97a9e3179@kloenk.dev
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>