can: flexcan: rename RX modes

Most flexcan IP cores support 2 RX modes:
- FIFO
- mailbox

The names for these modes were chosen to reflect the name of the
rx-offload mode they are using.

The name of the RX modes should better reflect their difference with
regards the flexcan IP core. So this patch renames the various
occurrences of OFF_FIFO to RX_FIFO and OFF_TIMESTAMP to RX_MAILBOX:

| FLEXCAN_TX_MB_RESERVED_OFF_FIFO -> FLEXCAN_TX_MB_RESERVED_RX_FIFO
| FLEXCAN_TX_MB_RESERVED_OFF_TIMESTAMP -> FLEXCAN_TX_MB_RESERVED_RX_MAILBOX
| FLEXCAN_QUIRK_USE_OFF_TIMESTAMP -> FLEXCAN_QUIRK_USE_RX_MAILBOX

Link: https://lore.kernel.org/all/20220107193105.1699523-4-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>