The function declarations for pci_cap_slot_get and
pci_cap_slot_write_config call the argument "slot_ctl", but the function
definitions and all the call sites drop the 'o' and call it "slt_ctl".
Let's be consistent.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>