Revert "dwc: re-rewrite barrier part"

Turns out we need all the barrier.
The one before writing the OWN bit is needed so the DMA engine
have coherent data, the second one (after the OWN bit is set) is
also needed because the DMA buffers are allocated with VM_MEMATTR_DEFAULT
whic equals to being cached.

This reverts commit 2b4f615e7d4720b16007cfa8aaedf720f572067a.
This reverts commit 4870346488051b9c4a1dc8b11cab55d4e5b7c230.

Reported by: mmel