net: dpaa2: fix and improve dpaa2-ptp driver

This patch is to fix and improve dpaa2-ptp driver
in some places.

- Fixed the return for some functions.
- Replaced kzalloc with devm_kzalloc.
- Removed dev_set_drvdata(dev, NULL).
- Made ptp_dpaa2_caps const.

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