Free Physical Address Mapping

Releases a mapping of physical to linear addresses that was
previously obtained with the Physical Address Mapping
function (Int 31H Fn 0800H).

Call With:

   AX     = 0801H
   BX:CX  = linear address returned by physical address
           mapping call

Returns:

   if function successful
   Carry flag= clear

   if function unsuccessful
   Carry flag= set
   AX     = error code
           8025H   invalid linear address

Notes:

o  The client should call this function when it is finished
   using a device previously mapped to linear addresses with
   the Physical Address Mapping function (Int 31H Function
   0800H).