PCI: tegra194: Rename 'root_bus' to 'root_port_bus' in tegra_pcie_downstream_dev_to_D0()

PCI: tegra194: Rename 'root_bus' to 'root_port_bus' in tegra_pcie_downstream_dev_to_D0()

In tegra_pcie_downstream_dev_to_D0(), PCI devices are transitioned to D0
state. For iterating over the devices, first the downstream bus of the Root
Port is searched from the root bus. But the name of the variable that holds
the Root Port downstream bus is named as 'root_bus', which is wrong.

Rename the variable to 'root_port_bus'. Also, move the comment on 'bringing
the devices to D0' to where the state is set exactly.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250922081057.15209-1-mani@kernel.org