tc: flower: make use of flower_port_attr_type() safe and silent

tc: flower: make use of flower_port_attr_type() safe and silent

Make use of flower_port_attr_type() safe:
* flower_port_attr_type() may return a valid index into tb[] or -1.
  Only access tb[] in the case of the former.
* Do not access null entries in tb[]

Also make usage silent - it is valid for ip_proto to be invalid,
for example if it is not specified as part of the filter.

Fixes: a1fb0d484237 ("tc: flower: Support matching on SCTP ports")
Signed-off-by: Simon Horman <simon.horman@netronome.com>