e1000e: ignore uninitialized checksum word on tgp

As described by Vitaly Lifshits:

> Starting from Tiger Lake, LAN NVM is locked for writes by SW, so the
> driver cannot perform checksum validation and correction. This means
> that all NVM images must leave the factory with correct checksum and
> checksum valid bit set.

Unfortunately some systems have left the factory with an uninitialized
value of 0xFFFF at register address 0x3F (checksum word location).
So on Tiger Lake platform we ignore the computed checksum when such
condition is encountered.

Signed-off-by: Jacek Kowalski <jacek@jacekk.info>
Tested-by: Vlad URSU <vlad@ursu.me>
Fixes: 4051f683 ("e1000e: Do not take care about recovery NVM checksum")
Cc: stable@vger.kernel.org
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Vitaly Lifshits <vitaly.lifshits@intel.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>