iwlegacy: collapse wrapper for pcie_capability_read_word()

il_pcie_link_ctl() has only one call site and no longer provides any useful
abstraction, so collapse it into the caller.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>