tty: convert "TTY Struct Flags" to an enum

tty: convert "TTY Struct Flags" to an enum

Convert TTY_* macros (flags) to an enum. This allows for easier
kernel-doc (the comment needed fine tuning), grouping of these nicely,
and proper checking.

Note that these are bit positions. So they are used such as
test_bit(TTY_THROTTLED, ...). Given these are not the user API (only
in-kernel API/ABI), the bit positions are NOT preserved in this patch.
All are renumbered naturally using the enum-auto-numbering.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>