clk: vt8500: Fix spinlock balance in vt8500_dclk_set_rate()

This function calls spin_lock_irqsave() twice, but never calls
spin_unlock_irqrestore(). Replace the second instance on
spin_lock_irqsave() with spin_unlock_irqrestore().

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>