board: phytec: check eeprom_data validity

board: phytec: check eeprom_data validity

For all of the functions that access the eeprom_data, make sure these
data are valid. Use the valid member of the phytec_eeprom_data struct.
This fixes a bug where only the API revision check guarded against
accessing rubbish. But if API revision was e.g. 6, eeprom setup failed
before, but phytec_get_imx8m_eth would still happily access the data.

Fixes: dc22188cdc8 ("board: phytec: Add common PHYTEC SoM detection")
Signed-off-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Daniel Schultz <d.schultz@phytec.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>