serial: geni_serial: Revert to FIFO only for now.

UART GPI DMA is broken for sm8150, frequently gives:
qcom_geni_serial c8c000.serial: serial engine reports 0 RX bytes in!
and
arm-smmu 15000000.iommu: Unhandled context fault: fsr=0x402, iova=0xfffc1080, fsynr=0x7a0013, cbfrsynra=0x7a3, cb=7