[FIX] account: wrong amount with _prepare_analytic_line

To have the right sign, amount = debit - credit

Before the fix:
 -Create an invoice in another currency than the currency of your company
 -Link the invoice lines to an analytic account.
 -Validate the invoice.

Bug:
The analytic lines have a negative amount in the amount currency field
which was a bit stange as it is the opposite sign than the amount in the company currency.

opw:692557

addons/account/models/account_move.py

@@ -1222,7 +1222,7 @@ class AccountMoveLine(models.Model):
         """ Prepare the values used to create() an account.analytic.line upon validation of an account.move.line having
             an analytic account. This method is intended to be extended in other modules.
         """
-        amount = (self.credit or 0.0) - (self.debit or 0.0)
+        amount = (self.debit or 0.0) - (self.credit or 0.0)
         return {
             'name': self.name,
             'date': self.date,