ixgbe: 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 ixgbe
driver still allowed TX packets to request HW timestamping. In this
situation, we see 'clearing Tx Timestamp hang' noise in the log.

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

Similar-to:
   commit 26bd4e2d ("igb: protect TX timestamping from API misuse")
   commit 0a6f2f05 ("igb: Fix a test with HWTSTAMP_TX_ON")

Signed-off-by: Manjunath Patil <manjunath.b.patil@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>