board:tricorder: always work with valid eeprom data

Commit 89088058 introduced EEPROM parsing and
board detection but faild to return a valid tricorder_eeprom struct for backup
case.  When pressing S200 while reading EEPROM we ignore the value. We
returned falsely a tricorder_eeprom struct with uninitialized data which is
just garbage.
Initialize it by zeroing the whole structure.

Signed-off-by: Andreas Bießmann <andreas.biessmann@corscience.de>
Cc: Thomas Weber <thomas.weber@corscience.de>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>