fru: ops: Do not let parser to write data to not allocated space

If customs fields in board area are used it will likely go over allocated
space in struct fru_board_data. That's why calculate limit of this
structure to make sure that different data is not rewritten by accident.
When limit is reached stop to record fields.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>