PCI: tegra: Fix ASPM-L1SS advertisement disable code

If the absence of CLKREQ# signal is indicated by the absence of
"supports-clkreq" in the device-tree node, current driver is disabling
the advertisement of ASPM-L1 Sub-States *before* the ASPM-L1 Sub-States
offset is correctly initialized. Since default value of the ASPM-L1SS
offset is zero, this is causing the Vendor-ID wrongly programmed to 0x10d2
instead of Nvidia's 0x10de thereby the quirks applicable for Tegra194 are
not being applied. This patch fixes this issue by refactoring the
code that disables the ASPM-L1SS advertisement.

Link: https://lore.kernel.org/r/20201203133451.17716-2-vidyas@nvidia.com
Fixes: 56e15a23

 ("PCI: tegra: Add Tegra194 PCIe support")
Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>