can/esd_usb: Rename all terms USB2 to USB

Each occurrence of the term "usb2" within variables, function names,
comments, etc. is changed to "usb" where it is shared for all
esd CAN/USB devices.

Link: https://lore.kernel.org/all/20220624190517.2299701-4-frank.jungclaus@esd.eu
Signed-off-by: Frank Jungclaus <frank.jungclaus@esd.eu>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>