tty: jsm_tty, make char+error handling readable

The code for char+error handling in jsm_input was complete mess of
letters. Introduce 3 new local variables and use them with care.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-21-jslaby@suse.cz


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