PCI: epf-mhi: Use iATU for small transfers

For transfers below 4K, let's use iATU since using eDMA for such small
transfers is inefficient.

This is mainly because setting up an eDMA transfer and waiting for
completion adds some latency. This latency is negligible for large
transfers but not for the smaller ones.

With using iATU, there is an increase in ~50Mbps throughput on both MHI
UL (Uplink) and DL (Downlink) channels.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20230717065459.14138-7-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>