tty: tty_ldisc: Do not use assignment in if condition

Do not use assignment in if condition following the advice of
checkpatch.pl.

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Link: https://lore.kernel.org/r/1617779210-51576-10-git-send-email-tanxiaofei@huawei.com


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