Fix the UNSUPPORTED argument handling in qt_ir_commandline_option

Set the argument name but not it's value in SHORT_NAME condition.
It works currently since the argument is visible from upper scope.

Pick-to: 6.7
Change-Id: Ie470a256f4735aeac5f50f0e6e5d17e2bb274ba2
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit f83837ec29)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

cmake/QtIRCommandLineHelpers.cmake

@@ -146,7 +146,7 @@ macro(qt_ir_commandline_option name)
         AND "${commandline_option_${arg_SHORT_NAME}_type}" STREQUAL "")
         set(unsupported "")
         if(arg_UNSUPPORTED)
-            set(unsupported "${arg_UNSUPPORTED}")
+            set(unsupported "UNSUPPORTED")
         endif()
 
         set(common "")