parport_pc locking fix

http://bugzilla.kernel.org/show_bug.cgi?id=8821 reports a might_sleep()
warning due to parport_pc_exit() running platform_device_unregister() while
holding ports_lock.

Just remove the locking: nobody else can access ports_list during module_exit.

Cc: "Mike Sharkey" <mike@pikeaero.com>
Cc: Tim Waugh <tim@cyberelk.net>
Cc: Stas Sergeev <stsp@aknet.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>