sparc: Clean arch-specific code in prom_common.c

prom_nextprop() and prom_firstprop() have slightly different calling
conventions in 32 and 64 bit SPARC.

prom_common.c uses a ifdef guard to ensure that these functions are
called correctly.

Adjust code to eliminate this ifdef by using a calling convention that
is compatible with both 32 and 64 bit SPARC.

Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>