rust: io: mem: enable IoRequest doc-tests

When introduced, the IoRequest doc-tests did depend on infrastructure
added in subsequent patches, hence they temporarily had to be disabled.

Now that we have the corresponding platform device infrastructure,
enable them.

Link: https://lore.kernel.org/r/DBG39YMN2TX6.1VR4PEQSI8PSG@kernel.org


Signed-off-by: Danilo Krummrich <dakr@kernel.org>