dpaa2-eth: Initialize mutex used in one step timestamping path

1588 Single Step Timestamping code path uses a mutex to
enforce atomicity for two events:
- update of ptp single step register
- transmit ptp event packet

Before this patch the mutex was not initialized. This
caused unexpected crashes in the Tx function.

Fixes: c5521189

 ("dpaa2-eth: support PTP Sync packet one-step timestamping")
Signed-off-by: Radu Bulie <radu-andrei.bulie@nxp.com>
Reviewed-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>