board: phytec: phytec_som_detection: fix eeprom_data zero check

In phytec_eeprom_data_init, after reading eeprom data into buffer, it is
checked whether all bytes are 0x0 by iterating over chunks of the
buffer. The offset, or index of the chunk, was never changed, leading to
repeated comparison of only the first chunk. Use array notation and
access chunk via array index to compare all chunks of the buffer.

Signed-off-by: Yannic Moog <y.moog@phytec.de>