can: esd_usb2: remove obsolete variable open_time

The variable open_time in the struct esd_usb2_net_priv was used to protect
esd_usb2_set_mode() only to be called, if the interface is up. Now the CAN
device infrastructure takes care of this.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>