net: pcnet: fix I/O primitives for memory access

Commit 69529c91 ("net: pcnet: Switch to PCI memory access")
switched from PCI IO access to PCI memory access without updating
the I/O primitives. Contrary to SH, the primitives for memory
access and IO access are implemented differently. Thus doing
memory access with IO port primitives breaks the driver on
MIPS Malta board.

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