can: pcan_usb: pcan_usb_fd_send_cmd(): silence compiler warning about uninitialized var

This patch silences the compiler warning:

drivers/net/can/usb/peak_usb/pcan_usb_fd.c: In function 'pcan_usb_fd_send_cmd':
drivers/net/can/usb/peak_usb/pcan_usb_fd.c:185:6: warning: 'err' may be used uninitialized in this function [-Wuninitialized]

by initialising the variable as 0.

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