kingst-la2016: rephrase FPGA bitstream content zero padding

kingst-la2016: rephrase FPGA bitstream content zero padding

The upload_fpga_bitstream() routine open codes a magic length for the
padded file content, which is hard to relate to a firmware file size,
and impossible to spot during maintenance. Declare an assumed chunk size
instead and let the resource file handling code path determine the needed
length of zero padding. Drop an unneeded device context member variable.