dpaa2-mac: Fix potential null pointer dereference

There is a null-check for _pcs_, but it is being dereferenced
prior to this null-check. So, if _pcs_ can actually be null,
then there is a potential null pointer dereference that should
be fixed by null-checking _pcs_ before being dereferenced.

Addresses-Coverity-ID: 1497159 ("Dereference before null check")
Fixes: 94ae899b ("dpaa2-mac: add PCS support through the Lynx module")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>