octeontx2-af: returning uninitialized variable

Fix coverity error 'use of uninitialized variable'. err is uninitialized
and is returned which can lead to unintended results. err has been replaced
with -einval.
Coverity issue: 1518921 (uninitialized scalar variable)

Signed-off-by: Sebin Sebastian <mailmesebin00@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>