mgb: support multiple segments in DMA

[br]

Summary

mgb: support multiple segments in DMA descriptor rings

not sure how did it work without this change before, I hit this straight away on PCI11414C

Test Plan

tested as part of D60539

[emaste]

Yeah it didn't work before, even from the initial commit

/*
 * XXX: This will be wrong in the multipacket case
 * I suspect FS should be for the first packet and
 * LS should be for the last packet
 */