tty: tty_driver: convert "TTY Driver Flags" to an enum

tty: tty_driver: convert "TTY Driver Flags" to an enum

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

Given these are flags, define them using modern BIT() instead of hex
constants.

It turns out (thanks, kernel-doc checker) that internal
TTY_DRIVER_INSTALLED was undocumented. Fix that too.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-14-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>