hdl/build_bladerf.sh: Fix SignalTap variable name

Fix erroneous variable name pointing to the SignalTap file to be used
when creating a Quartus project.

Author: bglod

hdl/quartus/build_bladerf.sh

@@ -434,7 +434,7 @@ quartus_sh --64bit \
            -projname "${PROJECT_NAME}" \
            -rev      "${rev}" \
            -flow     "${flow}" \
-           -stp      "${stp_file}" \
+           -stp      "${stp}" \
            -force    "${force}" \
            -seed     "${seed}"