common: Minor rework of tty_get.

* common/ttyio.c (do_get): Re-indent and remove the checking for char
values larger than 0xa0.  Use explicy control character checking.

The code is really old (mid 1998) and with the checking for 0xa0 it
has an implicit assumption of utf-8 or latin-1. Worse, the check was
for c > 0xa0 and not c == 0xa0 so it never worked as intended.

• Signed-off-by: Werner Koch <wk@gnupg.org>