TTY: jsm, remove superfluous check

data_len in jsm_input cannot be zero as we would jump out early in the
function. It also cannot be negative because it is an int and we do
bitwise and with 8192. So remove the check.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Lucas Tavares <lucaskt@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>