ipw2100: don't return positive values to PCI probe on error

Causes the PCI stack to complain, and then eventually call the
PCI remove function, which ipw2100 is not expecting.  It then
tries to unregister an already-released netdev and other nasty
things, leading to a panic.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1185518



Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>