powerpc/spufs: sputrace: Only enable logging on open(), prevent multiple openers

Currently, sputrace will start logging to the event buffer before the
log buffer has been open()ed. This results in a heap of "lost samples"
warnings if the sputrace file hasn't yet been opened.

Since the buffer is reset on open() anyway, there's no need to enable
logging when no-one has opened the log.

Because open clears the log, make it return EBUSY for mutliple open
calls.

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