selftests: mlxsw: Reduce runtime of tc-police scale test

Currently, the test takes about 626 seconds to complete because of an
inefficient use of the device's TCAM. Reduce the runtime to 202 seconds
by inserting all the flower filters with the same preference and mask,
but with a different key.

In particular, this reduces the deletion of the qdisc (which triggers
the deletion of all the filters) from 66 seconds to 0.2 seconds. This
prevents various netlink requests from user space applications (e.g.,
systemd-networkd) from timing-out because RTNL is not held for too long
anymore.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>