selftests: mlxsw: Generalize the parameters of mirror_gre test

Use the number of analyzers taken from the devlink command, instead of
hard-coded value, in order to make the test more generic.

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