cxgbe tom: Halve the size of offload transmit software descriptors

[jhb]

Summary

Use bitfields to pack tx_credits and plen into a single 32-bit word.

[jhb]

This isn't required for the other changes. Originally I was going to add another 1-bit field to struct ofld_tx_sdesc and had done this as a prep (I stole the extra bit from plen). However, I ended up not needing that change but still kept this as it does reduce memory usage a bit.