By Padraic Halpin and Tim Hepher DUBLIN, Jan 28 (Reuters) - A proposal by U.S. President Donald Trump to cap credit-card interest rates, if implemented, could have a big impact on the profitable loyalty programmes of ​the country's airlines, industry advisers said on Wednesday. Loyalty programmes have become a cash generator for carriers through the ‌sale of miles to third-party partners, mostly credit-card-issuing banks that award the miles to their own customers. The more customers spend, the more m ...