can: bittiming: can_validate_bitrate(): report error via netlink

Report an error to user space via netlink if the requested bit rate is
not supported by the device.

Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

drivers/net/can/dev/bittiming.c

···
 			return 0;
 	}
 
+	NL_SET_ERR_MSG_FMT(extack, "bitrate %u bps not supported",
+			   bt->brp);
+
 	return -EINVAL;
 }