PCI: designware: Simplify control flow

Return values immediately when possible to simplify the control flow.

No functional change intended.  Folded in unused variable removal as
pointed out by Fabio Estevam <fabio.estevam@nxp.com>, Arnd Bergmann
<arnd@arndb.de>, and Thierry Reding <thierry.reding@gmail.com>.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Pratyush Anand <pratyush.anand@gmail.com>