CVE-2026-72156: 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 headersize OOB read in mpfopsparseheader()

mpfopsparseheader() reads headersize from the bitstream at MPFHEADERSIZEOFFSET (24). When headersize is zero, the expression (buf + headersize - 1) reads one byte before the buffer start.

Since initialheadersize is set to 71 in mpfops, the fpga-mgr core guarantees the buffer is large enough to reach MPFHEADERSIZEOFFSET. The only real gap is the zero headersize case, which cannot be resolved by providing a larger buffer, so return -EINVAL.