selftests: spectrum-2: Adjust tc_flower_scale limit according to current counter count

With the change that made the code to query counter bank size from device
instead of using hard-coded value, the number of available counters
changed for Spectrum-2. Adjust the limit in the selftests.

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