can: uapi: update CAN-FD frame description

Since an early version of the CAN-FD specification the bit that
defines a CAN-FD frame on the wire, has been renamed from Extended
Data Length (EDL) to FD Frame (FDF).

To avoid confusion, update the struct canfd_frame description in the
UAPI headers accordingly.

Link: https://lore.kernel.org/r/20210517113727.77597-1-mkl@pengutronix.de


Suggested-by: Ayoub Kaanich <kayoub5@live.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>