pci: support both enable/enabled

Starting with linux kernel 3.13, the sysfs attribute 'enable' for pci
devices was renambed to 'enabled'.

Support the two notations to allow seamless migration.

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>
Merge pull request #27 from alonbl/pci

pci: support both enable/enabled