powerpc/spufs: Use simple_write_to_buffer

Simplify several write fileoperations for spufs by using
simple_write_to_buffer().

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>