ath9k: Use standard #defines for PCIe Capability ASPM fields

Use the standard #defines for PCIe Capability ASPM fields.

Previously we used PCIE_LINK_STATE_L0S and PCIE_LINK_STATE_L1 directly, but
these are defined for the Linux ASPM interfaces, e.g.,
pci_disable_link_state(), and only coincidentally match the actual register
bits.  PCIE_LINK_STATE_CLKPM, also part of that interface, does not match
the register bit.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>