driver core: Drop mention of obsolete bus rwsem from kernel-doc

15 years ago, commit 6eded061

 ("Fix up bus code and remove use of
rwsem") removed the bus rwsem, but left over a reference to it in a
kernel-doc comment.  Drop it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/b1af31b0e351bcbc056fe1ec44500737a7998d43.1594210157.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>