DPE: Fix compiler warnings

Fix warnings related to unused variables and unsigned to signed
pointer conversion. Also fix warning for possible missing return value
for non-void return type.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: Icd86127055e698d1ad57cb7f3649a8eb3580c326
4 files changed