serial: pch: Fix PCI device refcount leak in pch_request_dma()

In the Linux kernel, the following vulnerability has been resolved: serial: pch: Fix PCI device refcount leak in pch_request_dma() As comment of pci_get_slot() says, it returns a pci_device with its refcount increased. The caller must decrement the reference count by calling pci_dev_put(). Since 'dma_dev' is only used to filter the channel in filter(), we can call pci_dev_put() before exiting from pch_request_dma(). Add the missing pci_dev_put() for the normal and error path.