pci: Fix use of flags in dm_pci_map_bar()

The flags parameter of dm_pci_map_bar() is used for PCI region flags
rather than memory mapping flags. Fix the type to match that of the
region flags and stop using the regions flags as memory mapping flags.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>