pci: fixed dm_pci_map_bar comment

The comment now indicates that the input argument bar is a register offset,
not a BAR index.
It also mentions which BARs are supported for type 0/1 and that the
function can return 0 on error.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>