cxgb4: fix check for running offline ethtool selftest

The flag indicating the selftest to run is a bitmask. So, fix the
check. Also, the selftests will fail if adapter initialization has
not been completed yet. So, add appropriate check and bail sooner.

Fixes: 7235ffae ("cxgb4: add loopback ethtool self-test")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>