driver core: refcounting fix

Fix a reference counting bug exposed by commit
725522b5.  If driver.mod_name exists, we
take a reference in module_add_driver(), and never release it.  Undo that
reference in module_remove_driver().

Signed-off-by: Mike Galbraith <efault@gmx.de>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>