powerpc/spufs: Don't require full buffer in switch_log read

Currently, read() on the sputrace log will block until the read buffer
is full. This makes it difficult to retrieve the end of the buffer, as
the user will need to read with the right-sized buffer.

In a similar method as 91553a1b5e0df006a3573a88d98ee7cd48a3818a, this
change makes the switch_log return if there has already been data
read.

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