M.R.
I would try febreeze or dryer sheets as mentioned. I have also put a bar of soap in suitcases before & it helped to take away that musty smell. Just leave the bar of soap in it's pkg & it really helps. I also recently bought some air freshners in a pouch at Bed Bath & Beyond that are awesome. I bet you could drop 1 of those in the bag & it would get rid of that smell as well & they are around $5 for 3.