board:tricorder: always work with valid eeprom data

Commit 890880583d 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>

board/corscience/tricorder/tricorder.c

@@ -79,6 +79,8 @@ static void get_eeprom(struct tricorder_eeprom *eeprom)
 		} else {
 			panic("Could not get board revision\n");
 		}
+	} else {
+		memset(eeprom, 0, TRICORDER_EEPROM_SIZE);
 	}
 }