commit 4047b371 ("serial: core: Prevent unsafe uart port access,
part 1") added lockdep assertion for port mutex but fails to check
if debug_locks has disabled lockdep (so lock state is no longer
reliable).

Use lockdep_assert_held() instead, which properly checks lockdep
state as well.
Reported-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Tested-by: NTony Lindgren <tony@atomide.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>