cxgb4: use unaligned conversion for fetching timestamp

Use get_unaligned_be64() to fetch the timestamp needed for ns_to_ktime()
conversion.

Fixes following sparse warning:
sge.c:3282:43: warning: cast to restricted __be64

Fixes: a4569504 ("cxgb4: time stamping interface for PTP")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>