clk: at91: system: don't try to free_irq when there is no IRQ

In the error path of at91_clk_register_system(), sys->irq is freed
unconditionally but it may not exist or be request at all.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>