locking: fix mutex @key parameter kernel-doc notation

Fix @key parameter to mutex_init() and one of its callers.

Warning(linux-2.6.26-git11//drivers/base/class.c:210): No description found for parameter 'key'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>