[SPARC32]: Fix prom.c build warning

Fix these 2.6.19-rc1 build warnings:

  CC      arch/sparc/kernel/prom.o
arch/sparc/kernel/prom.c: In function `of_set_property':
arch/sparc/kernel/prom.c:246: warning: passing arg 2 of `prom_setprop' discards qualifiers from pointer target type
arch/sparc/kernel/prom.c: In function `build_one_prop':
arch/sparc/kernel/prom.c:446: warning: unused variable `len'
arch/sparc/kernel/prom.c:480: warning: ignoring return value of `prom_getproperty', declared with attribute warn_unused_result

Signed-off-by: Martin Habets <errandir_news@mph.eclipse.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>