Make the behaviour of clk_get_rate consistent with common clk's
clk_get_rate by accepting NULL clocks as parameter. Some device
drivers rely on this, and will cause an OOPS otherwise.

Fixes: 3f0a06b0 ("MIPS: ralink: adds clkdev code")
Reported-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Cc: John Crispin <john@phrozen.org>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16778/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>