can: peak_usb: pcan_usb_get_serial(): unconditionally assign serial_number

The function serial_number is only called from one location with a
valid serial_number pointer. Remove not needed NULL pointer check.

Link: https://lore.kernel.org/r/20210406111622.1874957-10-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>