MIPS: Fix wrong comments in asm/prom.h

In arch/mips/include/asm/prom.h, it should use "!CONFIG_USE_OF"
after #else and #endif.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>