fix: skip max discount validation for rate adjustment

(cherry picked from commit 5a3b133d65)

erpnext/controllers/selling_controller.py

@@ -28,6 +28,7 @@ class SellingController(StockController):
 	def validate(self):
 		super(SellingController, self).validate()
 		self.validate_items()
+		if not self.get("is_debit_note"):
 			self.validate_max_discount()
 		self.validate_selling_price()
 		self.set_qty_as_per_stock_uom()