serial/aspeed-vuart: fix a couple mod_timer() calls

The "unthrottle_timeout" is HZ/10 but mod_timer() takes a the actual
jiffie where you want it to timeout, not an offset.

Fixes: 5909c0bf ("serial/aspeed-vuart: Implement quick throttle mechanism")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>