PCI: return correct value when writing to the "reset" attribute

A successful write() to the "reset" sysfs attribute should return the
number of bytes written, not 0. Otherwise userspace (bash) retries the
write over and over again.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>