[TG3]: Add TG3_FLG2_IS_NIC flag.

Add Tg3_FLG2_IS_NIC flag to unambiguously determine whether the
device is NIC or onboard.  Previously, the EEPROM_WRITE_PROT flag was
overloaded to also mean onboard.  With the separation, we can
support some devices that are onboard but do not use eeprom write
protect.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>