pcnet-pci: Fix PIO word access to PROM

Implement the various IO access widths according to the spec. This
specifically unbreaks word and dword access to the PROM area that is
mapped into IO space. It also drops redundant upper limit checks and
spurious "return void".

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
