serial: samsung: protect NULL dereference of clock name

When priting the serial clock source, if clock source name is null,
kernel reference NULL point.

Signed-off-by: KeyYoung Park <keyyoung.park@samsung.com>
Signed-off-by: Huisung Kang <hs1218.kang@samsung.com>
Signed-off-by: Kyoungil Kim <ki0351.kim@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>