PNP: stop using the subsystem rwsem

The rwsem is not used to protect anything, so the use of it by the PNP
subsystem isn't really useful, and it's doubtful if it really did anything or
not.  So I've removed it.

Cc: Adam Belay <ambx1@neo.rr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>