fpga: Make formspec usage less prone to breakage

mesecons_fpga/init.lua

@@ -211,6 +211,7 @@ plg.from_formspec_fields = function(fields)
 			["="] = "buf",
 			["XNOR"] = "xnor",
 		}
+		s = s:gsub("^%s*", "") -- remove leading spaces
 		return mapping[s]
 	end
 	local is = {}