can: pcan_usb: don't provide CAN FD bittimings by non-FD adapters

The CAN FD data bittiming constants are provided via netlink only when there
are valid CAN FD constants available in priv->data_bittiming_const.

Due to the indirection of pointer assignments in the peak_usb driver the
priv->data_bittiming_const never becomes NULL - not even for non-FD adapters.

The data_bittiming_const points to zero'ed data which leads to this result
when running 'ip -details link show can0':

35: can0: <NOARP,ECHO> mtu 16 qdisc noop state DOWN mode DEFAULT group default qlen 10
    link/can  promiscuity 0
    can state STOPPED restart-ms 0
	  pcan_usb: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
	  : dtseg1 0..0 dtseg2 0..0 dsjw 1..0 dbrp 0..0 dbrp-inc 0  <== BROKEN!
	  clock 8000000

This patch changes the struct peak_usb_adapter::bittiming_const and struct
peak_usb_adapter::data_bittiming_const to pointers to fix the assignemnt
problems.

Cc: linux-stable <stable@vger.kernel.org> # >= 4.0
Reported-by: Oliver Hartkopp <socketcan@hartkopp.net>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>