PCI: Refactor pci_ioremap_bar() and pci_ioremap_wc_bar()

pci_ioremap_bar() and pci_ioremap_wc_bar() shared similar implementations
but differed in unimportant ways.  Align them by adding a shared helper,
__pci_ioremap_resource().

Upgrade warning message to error level, since it indicates a driver defect.
Remove WARN_ON() from WC path in favor of the error message.

[bhelgaas: commit log, use ioremap() since pci_iomap_range() doesn't add
anything]
Link: https://lore.kernel.org/r/20210713102436.304693-1-kw@linux.com


Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>