pci_virt_to_mem() uses virt_to_phys() to get the physical address.
But pci_virt_to_mem() is also called with uncached addresses which
is wrong according to the documentation of virt_to_phys().

Refactor the PCI_TO_MEM() macro to optionally map an uncached address
back to a cached one before calling pci_virt_to_mem().

Currently pcnet works because virt_to_phys() is incorrectly implemented
on MIPS. With the upcoming asm header file update for MIPS, the
virt_to_phys() implementation will be fixed. Thus this patch is needed
to keep pcnet working on MIPS Malta board.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>