tty: Remove baudrate dead code & make ktermios params const

tty: Remove baudrate dead code & make ktermios params const

With the architectures currently in-tree, either:
  1) CBAUDEX is zero
  2) The earlier BOTHER if check covers cbaud < 1 case
  3) All CBAUD bits are covered by the baud_table

Thus, the check for cbaud being out-of-range for CBAUDEX case cannot
ever be true.

The ktermios parameters can now be made const.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220816115739.10928-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>