[FIX] point_of_sale: no need to specify numeric digits

In the report pos order, average_price was set as a numeric(16,2), therefore, if the amount was too big, it led to a psql crash:
A field with precision 16, scale 2 must round to an absolute value less than 10^14.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64