SwiftLink driver: NMI disable during disk access

[Pinacolada64]

From @x-tec2017:

NMI is not getting disabled during serial port operation. If a program is loading and something comes on the SwiftLink, the program load will either get stuck and the drive activity light stays on, or if it manages to complete the load, the program that loaded will be corrupted. Try making the BBS load a large +.file like +.IM or +.EM and start hitting the Return key rapidly while logged on through the SwiftLink. That should duplicate the problem. The file rs232/swift.s has the routines for disabling and enabling NMI.