serial: sprd: Fix missing spin_unlock in sprd_handle_irq()

Fix return from sprd_handle_irq() with spin_lock held.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>