PCI: Remove unnecessary variable in pci_add_dynid()

The variable "retval" in pci_add_dynid() is only used to store the return
value of driver_attach() and is then directly returned.  Remove the
variable and directly pass on driver_attach()'s return value.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>