cxgb4: move PTP lock and unlock to caller in Tx path

Check for whether PTP is enabled or not at the caller and perform
locking/unlocking at the caller.

Fixes following sparse warning:
sge.c:1641:26: warning: context imbalance in 'cxgb4_eth_xmit' -
different lock contexts for basic block

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>