Support UMI packets with DW>256

Issue reported by @azaidy.

Switchboard packets can only directly convey UMI packets up to DW=256, but the UMI specification allows for DW=512 and DW=1024.  These larger data widths could be supported in at least two ways:

1. Split UMI packets with DW=512 or DW=1024 into multiple SB packets.
2. Increase the width of SB packets.