firmware/ci: Make '\0' char assignments consistent
firmware/ci: Make '\0' char assignments consistent

Null terminator character's were being set using 0, 0x00 and '\0'.
While these are all technically consistent we would like them to be
consistent as pointed out in a recent code review.

mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware,mithro/HDMI2USB-litex-firmware