sfxge(4): fix potential buffer overflow in Tx queue init

Improve error checking to avoid a caller overflowing the MCDI
request buffer if the requested TXQ size was excessively large.

Submitted by:   Andy Moreton <amoreton at solarflare.com>
Sponsored by:   Solarflare Communications, Inc.
MFC after:      1 week
Differential Revision:  https://reviews.freebsd.org/D18067

sys/dev/sfxge/common/ef10_tx.c

@@ -70,7 +70,7 @@ efx_mcdi_init_txq(
 	    EFX_TXQ_NBUFS(enp->en_nic_cfg.enc_txq_max_ndescs));
 
 	npages = EFX_TXQ_NBUFS(size);
-	if (npages > MC_CMD_INIT_TXQ_IN_DMA_ADDR_MAXNUM) {
+	if (MC_CMD_INIT_TXQ_IN_LEN(npages) > sizeof (payload)) {
 		rc = EINVAL;
 		goto fail1;
 	}