PCI/ASPM: Make sysfs link_state_store() consistent with link_state_show()

If CONFIG_PCIEASPM_DEBUG is set, then PCI devices have a link_state
attribute.  Reading that attribute shows the state as a bit mask: 1
means L0S upstream, 2 means L0S downstream, and 4 means L1.

Oddly, writing to link_state is inconsistent and gets translated, leading
to mysterious results in which the value you store isn't comparable the
value you load back out.

Fix it by making link_state_store() match link_state_show().

[bhelgaas: Check "aspm_disabled" *before* validating input.  When
"aspm_disabled" is set, this changes the error for invalid input from
-EINVAL to -EPERM.]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>