driver: soc: bam_dmux: Fix spinlock lock-up

Client invoking the BAM DMUX function from callback notifier and
calling the callback notifier by acquiring channel spinlock triggers
spinlock lock-up.

Remove the notifier callback from the spinlock context.

CRs-Fixed: 780192
Change-Id: Ic01c4d605510a68624dd9a14c35927892f1721d1
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>