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