msm: audio: qdsp6v2: Change acdb_mmap size variable to unsigned

The acdb_mmap fuction does a size calculation but
stores the result in an int which makes negitive
numbers possible. Changing the variable to unsigned
ensures that the size used is never out of bounds.

Change-Id: Iffd9c1440d2f28b703a77993fdbb3ccd18f30694
CRs-fixed: 548145
Signed-off-by: Ben Romberger <bromberg@codeaurora.org>