samples: rust_misc_device: Demonstrate additional get/set value functionality

Expand the complexity of the sample driver by providing the ability to
get and set an integer.  The value is protected by a mutex.

Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20241213134715.601415-4-lee@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>