PCI/ASPM: Remove redundant check of pcie_set_clkpm

Without supporting clock PM capable, if we want to disable clkpm, we don't
need this extra check as it must already be zero for the enable argument.
And it's the same for enabling clkpm here.  So let's remove this check.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>