Free Real Mode Callback Address

Releases a real mode callback address that was previously
allocated with the Allocate Real Mode Callback Address
function (Int 31H Fn 0303H).

Call With:

   AX     = 0304H
   CX:DX  = real mode callback address to be freed

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8024H   invalid callback address

Notes:

o  Real mode callbacks are a limited system resource.  A
   client should release any callback that it is no longer
   using.