serial: SERIAL_IFX6X60 should depend on HAS_DMA

If NO_DMA=y:

    drivers/built-in.o: In function `ifx_spi_free_device':
    ifx6x60.c:(.text+0x96d9a): undefined reference to `dma_free_coherent'
    drivers/built-in.o: In function `ifx_spi_spi_probe':
    ifx6x60.c:(.text+0x978a2): undefined reference to `dma_alloc_coherent'

While DMA is optional in this driver, and is used only if
ifx_modem_platform_data.use_dma is set, there are currently no in-tree
users of ifx_modem_platform_data (and thus of this driver), so just make
it depend on HAS_DMA.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>