selftests: mlxsw: mirror_gre: Simplify

After the previous patch, the function test_span_failable() is always
called with should_fail=1. Drop the argument and streamline the code.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>