igb: protect TX timestamping from API misuse

HW timestamping can only be requested for a packet if the NIC is first
setup via ioctl(SIOCSHWTSTAMP). If this step was skipped, then the igb
driver still allowed TX packets to request HW timestamping. In this
situation, the _IGB_PTP_TX_IN_PROGRESS flag was set and would never
clear. This prevented any future HW timestamping requests to succeed.

Fix this by checking that the NIC is configured for HW TX timestamping
before accepting a HW TX timestamping request.

Signed-off-by: Cliff Spradlin <cspradlin@google.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>