crypto: ice: Fix ICE key index type

ICE key index can be any positive integer, including value zero.
This makes key index default setting vulnerable to be set to zero.
Hence, making key index type as signed integer so that default
value of key index can be a negative number.

Change-Id: I234fd9335719490e604df407479e971a77a92efb
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>