pch_uart: don't hardcode PCI slot to get DMA device

The DMA is a function 0 of the multifunction device where SPI host is attached.
Thus, we may avoid to hardcode PCI slot number.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>