net: netxen: fix a missing check and an uninitialized use

When netxen_rom_fast_read() fails, "bios" is left uninitialized and may
contain random value, thus should not be used.

The fix ensures that if netxen_rom_fast_read() fails, we return "-EIO".

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>