rust: fix up formatting after merge

When I merged the rust 'use' imports, I didn't realize that there's
an offical preferred idiomatic format - so while it all worked fine,
it doesn't match what 'make rustfmt' wants to make it.

Fix it up appropriately.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>