hw/net/pcnet-pci: Unify pcnet_ioport_read/write and pcnet_mmio_read/write

hw/net/pcnet-pci: Unify pcnet_ioport_read/write and pcnet_mmio_read/write

The only difference between our implementation of the pcnet ioport
accessors and the mmio accessors is that the former check BCR_DWIO to
see what access widths are permitted for addresses in the aprom range
(0x0..0xf). In fact our failure to do this in the mmio accessors
is a bug (one which was fixed for the ioport accessors in
commit 7ba79741970 in 2011).

The data sheet for the Am79C970A does not describe the DWIO
bit as only applying for I/O space mapped I/O resources and
not memory mapped I/O resources, and our MMIO accessors already
honour DWIO for accesses in the 0x10..0x1f range (since the
pcnet_ioport_{read,write}{w,l} functions check it).

The data sheet for the later but compatible Am79C976 is clearer:
it states specifically "DWIO mode applies to both I/O- and
memory-mapped acceses." This seems to be reasonable evidence
in favour of interpretating the Am79C970A spec as being the same.

(NB: Linux's pcnet driver only supports I/O accesses, so the
MMIO access part of this device is probably untested anyway.)

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