pipeline: remove unreachable branch

pipeline: remove unreachable branch

A previous change blamed it on compiler/ASan, while this is a real
(yet minor) issue.

This return -EINVAL is never reached since we test all combinations of
fidx and fcin booleans.
All branches end up with a return 0, factorize them.

Fixes: 84f5ac9418ea ("pipeline: fix build with ASan")
Fixes: f38913b7fb8e ("pipeline: add meter array to SWX")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>