clkdev: fix clk_add_alias() with a NULL alias device name

clk_add_alias() was not correctly handling the case where alias_dev_name
was NULL: rather than producing an entry with a NULL dev_id pointer,
it would produce a device name of (null).  Fix this.

Cc: <stable@vger.kernel.org>
Fixes: 25689998 ("clkdev: add clkdev_create() helper")
Reported-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>