net/sched: taprio: don't allow dynamic max_sdu to go negative after stab adjustment

The overhead specified in the size table comes from the user. With small
time intervals (or gates always closed), the overhead can be larger than
the max interval for that traffic class, and their difference is
negative.

What we want to happen is for max_sdu_dynamic to have the smallest
non-zero value possible (1) which means that all packets on that traffic
class are dropped on enqueue. However, since max_sdu_dynamic is u32, a
negative is represented as a large value and oversized dropping never
happens.

Use max_t with int to force a truncation of max_frm_len to no smaller
than dev->hard_header_len + 1, which in turn makes max_sdu_dynamic no
smaller than 1.

Fixes: fed87cc6

 ("net/sched: taprio: automatically calculate queueMaxSDU based on TC gate durations")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>