cxgb4: Add HW timesptamp support for RX

Adds support for ethtool get time stamp ioctl, which is used by
tcpdump to get the supported time stamp types

eg: tcpdump -i eth5 -J
Time stamp types for eth5 (use option -j to set):
  host (Host)
  adapter_unsynced (Adapter, not synced with system time)

Adds support for adapter unsynced mode, by adding SIOCSHWTSTAMP support
in driver.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>