can: peak_usb: remove unused variables from struct peak_usb_device

This patch removes the unused variables struct
peak_usb_device::echo_skb and struct peak_usb_device::bus_load from
the driver.

Link: https://lore.kernel.org/r/20210406111622.1874957-4-mkl@pengutronix.de
Acked-by: Stephane Grosjean <s.grosjean@peak-system.com>
Tested-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>