staging: comedi: COMEDI_BUFINFO: update buffer before becoming non-busy

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 new
position.  For an asynchronous command in the "read" direction, if the
command has finished acquiring data normally, `do_become_nonbusy()` is
called to terminate the command.  That resets the buffer position, and
currently, the position information returned back to the user is after
the buffer has been reset.  It should be more useful to return the
buffer position before the reset, so move the call to
`do_become_nonbusy()` after the code that gets the updated buffer
position.

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