bus: mhi: host: pci_generic: Read SUBSYSTEM_VENDOR_ID for VF's to check status

In SR-IOV enabled devices, reading the VF DEVICE/VENDOR ID register
returns `FFFFh`, as specified in section 3.4.1.1 of the PCIe SR-IOV spec.
To accurately determine device activity, read the PCIe VENDOR_ID of
the Physical Function (PF) instead.
Health check monitoring for Virtual Functions (VFs) has been disabled,
since VFs are not physical functions and lack direct hardware control.
This change prevents unnecessary CPU cycles from being consumed by VF
health checks, which are both unintended and non-functional.

Signed-off-by: Vivek Pernamitta <quic_vpernami@quicinc.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Reviewed-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Link: https://patch.msgid.link/20250912-uevent_vdev_next-20250911-v4-2-fa2f6ccd301b@quicinc.com

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