n_tty: remove n_tty_receive_char_fast

n_tty_receive_char_fast is a copy of n_tty_receive_char with one
exception: PARMRK is not doubled in the former. Unify these two and
double PARMRK depending on a newly added parameter (bool parmrk_dbl).

I don't think the theoretical speedup is worth the code duplication.
Which is directly connected with maintenance burden.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-4-jslaby@suse.cz


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