sfxge(4): remove probes when a Tx queue is too full

No need for probe messages when a TxQ is too full for a post to be done.

Existing drivers check if there is room in the queue before posting
descriptors, even though efx_tx_qdesc_post() does the check itself.

The new SFN Windows driver doesn't perform the check before calling
efx_tx_qdesc_post(), but that means these probes can get frequently
printed out. It's normal driver behaviour so there's no need to print
an error.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18250