PCI: endpoint: epf-mhi: Use iATU for small transfers

For transfers below 4k, let's use iATU since using eDMA for such small
transfers is not efficient. This is mainly due to the fact that setting
up a eDMA transfer and waiting for its completion adds some latency. This
latency is negligible for large transfers but not for the smaller ones.

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

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>