tty: tty_buffer: unify tty_insert_flip_string_{fixed_flag,flags}()

They both do the same except for flags. One mem-copies the flags from
the caller, the other mem-sets to one flag given by the caller. This can
be unified with a simple if in the unified function.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-4-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>