can: peak_usb: pcan_usb_get_serial(): remove error message from error path

The caller of pcan_usb_get_serial() already prints an error message,
so remove this one and return immediately.

Link: https://lore.kernel.org/r/20210406111622.1874957-8-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>