drivers: can: npl: ensure CANFD_FDF flag is defined

Linux kernels before v5.14 do not define the CANFD_FDF flag. The flag is
disregarded within the Linux kernel. It is just there for being able to
reuse the canfd_frame struct for classic CAN frames in Linux user-space
applications.

Define the CANFD_FDF flag locally if not already defined to allow compiling
against older Linux kernel headers.

See the following Linux kernel commit for further details:
02546884221279da2725e87e35348290470363d7

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>