ptp_qoriq: check valid status before reading extts fifo

For PTP timer supporting external trigger timestamp FIFO,
there is a valid bit in TMR_STAT register indicating the
timestamp is available. For PTP timer without FIFO, there
is not TMR_STAT register.
This patch is to check the valid bit for the FIFO before
reading timestamp, and to avoid operating TMR_STAT register
for PTP timer without the FIFO.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>