macb: add RM9200's interrupt flag TBRE

Transmit Buffer Register Empty replaces TXERR on RM9200 and signals the
sender may try to send again becase the last queued frame is no longer
in queue (being transmitted or already transmitted).

Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Claudiu Beznea <claudiu.beznea@microchip.com>
Cc: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20201011090944.10607-2-w@1wt.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/cadence/macb.h

@@ -366,6 +366,8 @@
 #define MACB_ISR_RLE_SIZE	1
 #define MACB_TXERR_OFFSET	6 /* EN TX frame corrupt from error interrupt */
 #define MACB_TXERR_SIZE		1
+#define MACB_RM9200_TBRE_OFFSET	6 /* EN may send new frame interrupt (RM9200) */
+#define MACB_RM9200_TBRE_SIZE	1
 #define MACB_TCOMP_OFFSET	7 /* Enable transmit complete interrupt */
 #define MACB_TCOMP_SIZE		1
 #define MACB_ISR_LINK_OFFSET	9 /* Enable link change interrupt */