powerpc/spufs: Fix test in spufs_switch_log_read()

size_t len cannot be less than 0.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>