cmd: tlv_eeprom: enable 'dev' subcommand before 'read'

Move the handler for "tlv_eeprom dev X" command to the beginning of
do_tlv_eeprom, to allow using it before issuing a "read" command for
currently selected eeprom.

Also remove the check if eeprom exists, since that can only work after
the first execution of read_eeprom triggered device lookup.
Instead accept values up to the defined array size (MAX_TLV_DEVICES).

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>