ixgbe: Do not schedule an uninitialized workqueue entry

If a hardware Tx timestamp is requested, an uninitialized
workqueue entry may be scheduled, especially on an 82598 adapter.
Add a check for a PTP clock to avoid that. Also only apply the
unlikely to the first term of the conditional. That will make the
rest of the checks be in the cold path.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Acked-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>