dev: Remove the return type from DmaPort::dmaAction.

dev: Remove the return type from DmaPort::dmaAction.

This function had a comment claiming that returning an arbitrary request
from the call was necessary for page table walker statistics, but
looking at the actual code, the return type was never used. Also
returning whatever the last request happens to be seems arbitrary, and a
bad boundary for modularization. The page table walker should not depend
on the internal implementation of the DMA port.

Change-Id: I00281fbaf6aeb85b15baf54f3d4a23ca1ac72b8b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38716
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>