can: usb: Kconfig: Add PCAN-USB X6 device in help text

This patch adds a text line in the help section of the CAN_PEAK_USB
config item describing the support of the PCAN-USB X6 adapter, which is
already included in the Kernel since 4.9.

Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>