db_update add value to field
The problem is way I used to update,
for this job drupal has syntax, according to  Update query with a dynamic expression in Drupal 7?
I found my solution 

$cr_update_no= db_update('rah_cc_credit') // Table name no longer needs {}
                -&gt;fields(array('uid' =&gt; $user-&gt;uid,'update_date'=&gt;REQUEST_TIME,'last_code'=&gt;$code ))
                -&gt;expression('credit', 'credit + :amount', array(':amount' =&gt; $credit))
                -&gt;condition('uid', $user-&gt;uid, '=')
                -&gt;execute();


I submit this answer maybe helpful for another in future.