x86/PCI: Fix Broadcom CNB20LE unintended sign extension

In the expression "word1 << 16", word1 starts as u16, but is promoted to
a signed int, then sign-extended to resource_size_t, which is probably
not what was intended.  Cast to resource_size_t to avoid the sign
extension.

Found by Coverity (CID 138749, 138750).

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>