macb: add RM9200's interrupt flag TBRE

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>