tty: make tty const in tty_get_baud_rate()

tty: make tty const in tty_get_baud_rate()

After commit 87888fb9ac0c ("tty: Remove baudrate dead code & make
ktermios params const"), the 'tty' parameter is only read in
tty_get_baud_rate(). Therefore, we can make 'tty' accepted in the
function 'const' for clarity.

The "the terminal bit flags may be updated." part of the
tty_get_baud_rate()'s kernel-doc is dropped as it is no longer true.
Because of the same commit above. And it was misplaced anyway.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20231127123713.14504-1-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>