net: dsa: mv88e6xxx: hwtstamp: remove unnecessary range checking tests

_port_ is already known to be a valid index in the callers [1]. So
these checks are unnecessary.

[1] https://lkml.org/lkml/2018/2/16/469



Addresses-Coverity-ID: 1465287
Addresses-Coverity-ID: 1465291
Suggested-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>