Add additional error handling to pcidev_readbar() callers.

This is mostly a leftover of Niklas' "remove exit call from pcidev_init" patch.
While not explicitly necessary detecting errors early is usually a good idea.

Signed-off-by: Niklas Söderlund <niso@kth.se>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1718 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1