e1000e: don't return half-read eeprom on error

On a read error, e1000e might have returned uninitialized block of
eeprom data back to userspace. The convention is that 0xff is "empty",
so mark the entire eeprom as empty in case of an error.

Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>