tty: serial: fix help message for SERIAL_CPM

Running checkpatch reported error extra whitespace at the
end of help message for SERIAL_CPM menu entry

Signed-off-by: Arturs Artamonovs <Arturs.Artamonovs@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>