net/mlx4: compact flow rule error reporting

net/mlx4: compact flow rule error reporting

Relying on rte_errno is not necessary where the return value of
rte_flow_error_set() can be used directly.

A related minor change is switching from RTE_FLOW_ERROR_TYPE_HANDLE to
RTE_FLOW_ERROR_TYPE_UNSPECIFIED when no rte_flow handle is involved in the
error, specifically when none is allocated yet.

This commit does not cause any functional change.

Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Acked-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>