ASoC: msm: 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: Ie3cf4c0a1037e7f5fef29b8301becc6c19200ed6
CRs-fixed: 548145
Signed-off-by: Ben Romberger <bromberg@codeaurora.org>