Whenever there is production of sputum, it is productive - and not dry - cough. Codeine is produced from opium and is indicated only in dry cough. Furthermore its cough suppressant property leads to sputum retention and makes asthma worse. Hence codeine is not given to asthmatics. There is no data on the safety of codeine during pregnancy and it is best avoided in the first three months.
There is no mention of the continued use of Seroflo 125 in your communication. You should continue it. There is no problem during pregnancy.
With regard to the sputum, the more it comes out, the better. I suggest you do steam inhalation. Simple face inhalers - used in beauty parlours - are available at most chemist shops and cost around Rs. 150-200.