ocxl: fix pci device refcount leak when calling get_function_0()

In the Linux kernel, the following vulnerability has been resolved:

ocxl: fix pci device refcount leak when calling get_function_0()

get_function_0() calls pci_get_domain_bus_and_slot(), as comment
says, it returns a pci device with refcount increment, so after
using it, pci_dev_put() needs be called.

Get the device reference when get_function_0() is not called, so
pci_dev_put() can be called in the error path and callers
unconditionally. And add comment above get_dvsec_vendor0() to tell
callers to call pci_dev_put().