fpga: microchip-spi: fix zero header_size OOB read in mpf_ops_parse_header()

In the Linux kernel, the following vulnerability has been resolved: fpga: microchip-spi: fix zero header_size OOB read in mpf_ops_parse_header() mpf_ops_parse_header() reads header_size from the bitstream at MPF_HEADER_SIZE_OFFSET (24). When header_size is zero, the expression *(buf + header_size - 1) reads one byte before the buffer start. Since initial_header_size is set to 71 in mpf_ops, the fpga-mgr core guarantees the buffer is large enough to reach MPF_HEADER_SIZE_OFFSET. The only real gap is the zero header_size case, which cannot be resolved by providing a larger buffer, so return -EINVAL.