net: pcnet: use pci_virt_to_mem to obtain buffer addresses

The pcnet driver uses the pci_phys_to_mem function
to get the memory address of the DMA buffers. This
This assumes an 1:1 mapping between the PCI and
physical memory which is not true on all platforms.

On MIPS platform U-Boot is running within a mapped
memory region, and the pci_phys_to_mem macro can't
be used to obtain the memory address of the buffers.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>