staging: comedi: COMEDI_BUFINFO: terminate "write" command when stopped

The `COMEDI_BUFINFO` ioctl is used to advance the current position in
the buffer by a specified amount (which can be 0) and get the current
position.  An asynchronous command in the "read" direction is terminated
automatically once it has stopped and information about the final
position and error has been reported back to the user.  That is not
currently done for commands in the "write" direction.  Change it to
terminate the command in the "write" direction automatically.  If the
command stopped with an error, report an `EPIPE` error back to the user,
otherwise just report the final buffer position back to the user.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>