niu: fix missing checks of niu_pci_eeprom_read

niu_pci_eeprom_read() may fail, so we should check its return value
before using the read data.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Acked-by: Shannon Nelson <shannon.lee.nelson@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>