bitstream_writer: fix 7 series and US+ headers
bitstream_writer: fix 7 series and US+ headers

Signed-off-by: Karol Gugala <38af71640571ee4b5093d4f69ffca1ac454f2948@antmicro.com>

SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray,SymbiFlow/prjxray