can: kvaser_usb: Split driver into kvaser_usb_core.c and kvaser_usb_leaf.c

First part of adding support for Kvaser USB device family "hydra".

Split kvaser_usb.c into kvaser_usb/kvaser_usb{.h,_core.c,_leaf.c}.

kvaser_usb_core.c contains common functionality, such as USB
writing/reading and allocation of netdev.
kvaser_usb_leaf.c contains device specific code, used in
kvaser_usb_core.c.

struct kvaser_usb_dev_ops contains device specific functions that are
common for all devices in the family. While, struct kvaser_usb_dev_cfg
describes the device configurations in terms of CAN clock frequency,
timestamp frequency and CAN controller bittiming constants.

Signed-off-by: Jimmy Assarsson <extja@kvaser.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>