[efi] Poll for TX completions only when there is an outstanding TX buffer

At least one NII implementation (in a Microsoft Surface tablet) seems
to fail to report the absence (sic) of TX completions properly.  Work
around this by checking for TX completions only when we expect to see
one.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/efi/nii.c

 		return;
 
 	/* Sanity check */
-	if ( ! nii->txbuf ) {
-		DBGC ( nii, "NII %s reported spurious TX completion\n",
-		       nii->dev.name );
-		netdev_tx_err ( netdev, NULL, -EPIPE );
-		return;
-	}
+	assert ( nii->txbuf != NULL );
 
 	/* Complete transmission */
 	iobuf = nii->txbuf;
 	/* Get status */
 	op = NII_OP ( PXE_OPCODE_GET_STATUS,
 		      ( PXE_OPFLAGS_GET_INTERRUPT_STATUS |
-			PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS |
+			( nii->txbuf ? PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS : 0)|
 			( nii->media ? PXE_OPFLAGS_GET_MEDIA_STATUS : 0 ) ) );
 	if ( ( stat = nii_issue_db ( nii, op, &db, sizeof ( db ) ) ) < 0 ) {
 		rc = -EIO_STAT ( stat );
 	}
 
 	/* Process any TX completions */
-	nii_poll_tx ( netdev, stat );
+	if ( nii->txbuf )
+		nii_poll_tx ( netdev, stat );
 
 	/* Process any RX completions */
 	nii_poll_rx ( netdev );