msm: sps: add the checking of ZLT in case of an empty pipe

Check if a ZLT descriptor is the last pending descriptor in
the desc FIFO when determine if the desc FIFO of that pipe is
empty.

Change-Id: Ib8174953d6ad7d886f47f21e243e985b79ef41fb
Signed-off-by: Yan He <yanhe@codeaurora.org>