Fix alignment of RX bufs DMA map to be realistic.  Maybe it works on i386 (I don't know) but on amd64 at hand here, it paniced early at boot.
Fix alignment of RX bufs DMA map to be realistic.  Maybe it works on
i386 (I don't know) but on amd64 at hand here, it paniced early at
boot.

(I'm pretty sure that PAGE_SIZE here was miscopied from another place
during porting, where in OpenBSD bus_dmamem_alloc() is used, but there
PAGE_SIZE means completely different thing.)

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