usb: hso: check for return value in hso_serial_common_create()

in case of an error tty_register_device_attr() returns ERR_PTR(),
add IS_ERR() check

Reported-and-tested-by:  <syzbot+67b2bd0e34f952d0321e@syzkaller.appspotmail.com>
Link: https://syzkaller.appspot.com/bug?extid=67b2bd0e34f952d0321e


Signed-off-by: Rustam Kovhaev <rkovhaev@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>