cmd: tlv_eeprom: handle -ENODEV error from read_eeprom function

When tlv eeprom does not exist, return error code instead of quietly
making up tlv structure in memory.

Signed-off-by: Josua Mayer <josua@solid-run.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>