powerpc/spufs: sputrace: Don't block until the read buffer is full

Currently, read() on the sputrace buffer will only return data when
the user buffer is exhausted. This may mean that we never see the
end of the event log, unless we read() with exactly the right-sized
buffer.

This change makes sputrace_read not block if we have data ready to
return.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>