cxgbe tom: Halve the size of offload transmit software descriptors

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

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47759

(cherry picked from commit 5a38857684907c52982787dbac2c5e5c8abfd4f8)