msm: ipa: Fix memory free issue

When calling dma_free_coherent, there is a possibility that
size 0 is passed as argument which can result into panic.
Make a change to check for the size before calling the API.

Change-Id: Idb248e1270b79b51e53de687793030a047fe6c73
Acked-by: Chaitanya Pratapa <cpratapa@qti.qualcomm.com>
Signed-off-by: Ravinder Konka <rkonka@codeaurora.org>