PCI: Restrict VPD read permission to root

Some PCI devices will lock up if we attempt to read from VPD addresses
beyond some device-dependent limit.  Until we can identify these
devices and adjust the file size accordingly, only let root read VPD
through sysfs to prevent a DoS by normal users.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>