rust: helpers: Remove volatile qualifier from io helpers

Remove the `volatile` qualifier used with __iomem in helper functions
in io.c. These helper functions are just wrappers around the
corresponding accessors so they are unnecessary.

This fixes the following UML build error with CONFIG_RUST enabled:

In file included from rust/helpers/helpers.c:19:
rust/helpers/io.c:12:10: error: passing 'volatile void *' to parameter of type 'void *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
   12 |         iounmap(addr);
      |                 ^~~~
arch/um/include/asm/io.h:19:42: note: passing argument to parameter 'addr' here
   19 | static inline void iounmap(void __iomem *addr)
      |                                          ^
1 error generated.

[ Arnd explains [1] that removing the qualifier is the way forward
  (thanks!):

    Rihgt, I tried this last week when it came up first, removing the
    'volatile' annotations in the asm-generic/io.h header and then
    all the ones that caused build regressions on arm/arm64/x86
    randconfig and allmodconfig builds.  This patch is a little
    longer than my original version as I did run into a few
    regressions later.

    As far as I can tell, none of these volatile annotations have
    any actual effect, and most of them date back to ancient kernels
    where this may have been required.

    Leaving it out of the rust interface is clearly the right way,
    and it shouldn't be too hard to upstream the changes below
    when we need to, but I also don't see any priority to send these.
    If anyone wants to help out, I can send them the whole patch.

  I created an issue [2] in case someone wants to help. - Miguel ]

Fixes: ce30d94e ("rust: add `io::{Io, IoRaw}` base types")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Cc: stable@vger.kernel.org
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/0c844b70-19c7-4b14-ba29-fc99ae0d69f0@app.fastmail.com/ [1]
Link: https://github.com/Rust-for-Linux/linux/issues/1156 [2]
Link: https://lore.kernel.org/r/20250412005341.157150-1-fujita.tomonori@gmail.com


[ Reworded for relative paths. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>