mdp3: ppp: simplify ppp completion logic

This fixes issue where ppp would no longer process requests
claiming that ppp core is hung, when in fact ppp core was
not active at all.

Change-Id: I257858a61df5b3141d835ad6c6fff78c72bf7cbd
Signed-off-by: Terence Hampson <thampson@codeaurora.org>