net: pcnet: Replace mips-specific accessors

Replace mips-specific UNCACHED_SDRAM() macro with standard
map_physmem(), which permits the driver to work on other
systems than mips.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>