PCI: designware: Remove open-coded bitmap operations

Replace them by using the standard kernel bitmap ops.  No functional
change, but makes the code a lot cleaner.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pratyush Anand <pratyush.anand@st.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>