msm: sps: use a new flag for disconnecting pipe

Use a new flag to indicate if BAM client has decided to disconnect
a pipe so that we can avoid BAM IRQ handling for a disconnected
pipe.

Change-Id: I1d1e467cf7366f0b00b32674117d381432611e26
Signed-off-by: Yan He <yanhe@codeaurora.org>