ptp_qoriq: fix interrupt enabling and handling

The tmr_tevent register would update event bits
no matter tmr_temask bits were set or not. So we
should get interrupts by tmr_tevent & tmr_temask,
and clean up interrupts in tmr_tevent before
enabling them.

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