tty: xuartps: Remove '_OFFSET' suffix from #defines

Remove the _OFFSET suffix from all register defines which makes code a
little easier to read and avoids a few line breaks.
Suggested-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>