ada: Spurious maximum nesting level warnings

This patch fixes an issue in the compiler whereby disabling style checks via
pragma Style_Checks ("-L") resulted in the minimum nesting level being zero
but the style still being enabled - leading to spurious maximum nesting level
exceeded warnings.

gcc/ada/

	* stylesw.adb (Set_Style_Check_Options): Disable max nesting level
	when unspecified

gcc/ada/stylesw.adb

@@ -537,7 +537,8 @@ package body Stylesw is
                Style_Check_Layout                 := False;
 
             when 'L' =>
-               Style_Max_Nesting_Level := 0;
+               Style_Max_Nesting_Level            := 0;
+               Style_Check_Max_Nesting_Level      := False;
 
             when 'm' =>
                Style_Check_Max_Line_Length        := False;