drivers: ethernet: tc6: Check footer parity before updating struct oa_tc6

The parity of the received footer from data transfer (also including the
NORX) shall be checked before members of struct tc6 are updated.

This prevents from updating the driver's crucial metadata (i.e. struct
oa_tc6) with malformed values and informs the upper layers of the driver
that error has been detected.

Signed-off-by: Lukasz Majewski <lukma@denx.de>