dsp: Fix rtac memory unmap issue in ASM driver

During unmap of rtac block in ASM, mem_map_handle
address is set to zero instead of the value. Set the
map handle value to zero to avoid issue in freeing the
ion memory.

CRs-Fixed: 2254339
Change-Id: I6584be029d4c8dde235e722149c758df0db9916e
Signed-off-by: Aditya Bavanari <abavanar@codeaurora.org>