fx2lafw: silence integer overflow compile warnings

fx2lafw: silence integer overflow compile warnings

Explicitly mark literals for control bitfield values as unsigned. This
avoids a compiler warning when the MSB gets set in assignments to a BYTE.

The warning was not seen with SDCC v3.3, but with v3.6.9:

  SDCC     fx2lafw.rel
  ./source/fx2lafw.c:66: warning 158: overflow in implicit constant conversion
  SDCC     gpif-acquisition.rel
  ./source/gpif-acquisition.c:65: warning 158: overflow in implicit constant conversion