Remove spurious warning about invalid VPD data.

The warning used to be displayed for valid VPDs about 512B or above in
size. Fix the size check and add a break while here so that the routine
stops if if detects any problem.

Tested with "pciconf -lV"

Reviewed by: kib@, jhb@
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D23679