[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

@@ -785,12 +785,7 @@ static void nii_poll_tx ( struct net_device *netdev, unsigned int stat ) {
 		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;
@@ -888,7 +883,7 @@ static void nii_poll ( struct net_device *netdev ) {
 	/* 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 );
@@ -898,7 +893,8 @@ static void nii_poll ( struct net_device *netdev ) {
 	}
 
 	/* 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 );