sfxge(4): limit max TXQ size on Medford to 2048

Queues with 4096 descriptors are not supported as the top bit is used for vfifo
stuffing.

Submitted by: Mark Spender <mspender at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D8948