asoc: wcd-dsp-mgr: clear work queue for wcd dsp manager

wdsp_mgr_bind would schedule work to load wdsp firmware.
Before loading wdsp firmware, sound card registeration may fail.
wcd_dsp_cntl_deinit would get called to free cntl which would be used
in wdsp_load_fw_image which would cause kernel panic.
Clear work queue in wdsp_mgr_unbind to avoid kernel panic.

Change-Id: I6c052fa8ea1ff62a8b075ba191085612242a5005
Signed-off-by: Meng Wang <mwang@codeaurora.org>