[FIX] orm: do not recompute m2o stored function field at read.

The get() method of m2o function fields is used for 2 different things:
 - call the function defining the m2o
 - get the name_get representation of the value

Until this pathc, only the first case was handled, resulting to a useless
recomputation of the field when reading it.
Merge pull request #404 from odoo-dev/7.0-norecomputestoredm2ofuncfields-chs

[FIX] orm: do not recompute m2o stored function field at read.