can: netlink: add can_validate_databittiming()

can: netlink: add can_validate_databittiming()

Factorise the databittiming validation out of can_validate() and move
it in the new add can_validate_databittiming() function. Also move
can_validate()'s comment because it is specific to CAN FD. This is a
preparation patch for the introduction of CAN XL as this databittiming
validation will be reused later on.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Link: https://patch.msgid.link/20250923-canxl-netlink-prep-v4-6-e720d28f66fe@kernel.org
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>