driver-core: fix kernel-doc parameter name

Fix function parameter name in kernel-doc:

Warning(linux-next-20090120//drivers/base/core.c:1289): No description found for parameter 'dev'
Warning(linux-next-20090120//drivers/base/core.c:1289): Excess function parameter 'root' description in 'root_device_unregister'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>