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.