clk: vt8500: Fix unbalanced spinlock in vt8500_dclk_set_rate()

With the addition of a DVO clock, a bug is now evident in the vt8500
clock code:
[    0.290000] WARNING: at init/main.c:698 do_one_initcall+0x158/0x18c()
[    0.300000] initcall wm8505fb_driver_init+0x0/0xc returned with disabled int

This is caused by an unbalanced spinlock in vt8500_dclk_set_rate().
Replace the second call to spin_lock_irqsave() with spin_unlock_irqrestore().

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Mike Turquette <mturquette@linaro.org>