hw/net/pcnet-pci: Convert away from old_mmio accessors

hw/net/pcnet-pci: Convert away from old_mmio accessors

Convert the pcnet-pci device away from using the old_mmio
MemoryRegionOps accessor functions.

This commit is a no-behaviour-change API conversion.
(Since PCNET_PNPMMIO_SIZE is 0x20, the old "addr & 0x10"
check and the new "addr < 0x10" check are exact opposites;
the new code is phrased to be parallel with the
pcnet_io_read/write functions.)

I have left a TODO comment marker because the similarity
between the MMIO and IO accessor behaviour is suspicious
and they could be combined, but this will be left to a
different patch.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>