net: usb: lan78xx: Fix error message format specifier

commit 858ce8ca upstream.

Display the return code as decimal integer.

Fixes: 55d7de9d ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Signed-off-by: Cristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>