Refactor validation handler allowing add to cart by product SKU or title

The autocompletion allows to search a product by SKU or Title but if you enter the product title directly this product will be invalidated by the commerce_quick_purchase_validate_sku() despite the fact it is a valid product.

Conclusion: Allow adding products to cart both by SKU and Title refactoring the validation function into the commerce_quick_purchase_validate_value($value).