serial: ifx6x60: dma_alloc_coherent must use parent dev

This driver is a SPI protocol driver and has no DMA ops
associated with the device so the call will fail. Furthermore,
the DMA allocation made here will be used by the SPI
controller driver (parent dev) so it makes sense to
pass that device instead.

Signed-off-by: Russ Gorby <russ.gorby@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>