can: mcp251xfd: mcp251xfd_start_xmit(): use mcp251xfd_get_tx_free() to check TX is is full

This patch replaces an open coded check if the TX ring is full by a check if
mcp251xfd_get_tx_free() returns 0.

Link: https://lore.kernel.org/r/20210114153448.1506901-2-mkl@pengutronix.de


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