can: mcp251xfd: introduce and make use of mcp251xfd_is_fd_mode()

This patch replaces the open coded check, if the chip's FIFOs are
configured for CAN-FD mode, by the newly introduced function
mcp251xfd_is_fd_mode().

Link: https://lore.kernel.org/all/20220105154300.1258636-14-mkl@pengutronix.de


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