PCI_READ_VIA_DWORD_OP: Fix *val uninitialized bug

This patch has been sent on:
- 6 Jun 2007

Many users of PCI config read routines tend to ignore the function
ret value, and are only concerned about the contents of *val. Based
on this, pci_hose_read_config_{byte,word}_via_dword should initialize
the *val on dword read error.

Without this fix, for example, we'll go on scanning bus with vendor or
header_type uninitialized. This brings many unnecessary config trials.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>