你可以给hiring manager (and/or team leader)发个email,

一般interview完不是都要发Email道谢的吗，你正好利用这个机会再顺便assure them that you won't have visa problems.

The email can go something like this:

I wanted to thank you again for taking the time to interview with me about this position. ...blah blah....

I would also like to take this opportunity to further clarify my visa status. .... (如果你有帮你办绿卡的移民律师，你可以把他/她的联系电话附上，说明in case they have questions please contact this lawyer.)

Email 最好简洁明了，不要太长（尤其不要解释很多细节)，just make your point.

Good luck -- hope you get that offer.