dpaa_eth: avoid timestamp read on error paths

The dpaa_cleanup_tx_fd() function is called by the frame transmit
confirmation callback but also on several error paths. This function
is reading the transmit timestamp value. Avoid reading an invalid
timestamp value on the error paths.

Fixes: 4664856e ("dpaa_eth: add support for hardware timestamping")
Signed-off-by: Madalin Bucur <madalin.bucur@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>