[POWERPC] spufs: Read from signal files only if data is there

We need to check the channel count of the signal notification registers
before reading them, because it can be undefined when the count is
zero. In order to read count and data atomically, we read from the
saved context.

This patch uses spu_acquire_saved() to force a context save before a
/signal1 or /signal2 read. Because of this it is no longer necessary to
have backing_ops and hw_ops versions of this function so they have been
removed.

Regular applications should not rely on reading this register
to be fast, as it's conceptually a write-only file from the PPE
perspective.

Signed-off-by: Dwayne Grant McConnell <decimal@us.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>