tty: Move TIOCSTI toggle variable before kerndoc

tty: Move TIOCSTI toggle variable before kerndoc

The variable "tty_legacy_tiocsti" should be defined before the kerndoc
for the tiocsti() function. The new variable was breaking the "htmldocs"
build target:

drivers/tty/tty_io.c:2271: warning: cannot understand function prototype: 'bool tty_legacy_tiocsti __read_mostly = IS_ENABLED(CONFIG_LEGACY_TIOCSTI); '

Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/lkml/20221107143434.66f7be35@canb.auug.org.au
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20221107034631.never.637-kees@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>