PCI/ASPM: Use standard parsing functions for sysfs setters

The functions link_state_store() and clk_ctl_store() had just subtracted
ASCII '0' from input which could lead to undesired results.  Instead, use
Linux string functions to safely parse input.

[bhelgaas: check kstrtouint() return value]
Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>