fm10k: report the receive timestamp in FM10K_CB(skb)->tstamp

This was accidentally removed when we defeatured the full 1588 Clock
support. We need to report the Rx descriptor timestamp value so that
applications built on top of the IES API can function properly.

Additionally, remove the FM10K_FLAG_RX_TS_ENABLED, as it is not used now
that 1588 functionality has been removed.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>