dm-req-crypt: Remove high priority flag from workqueue

Remove WQ_HIGHPRI flag for the workqueue. WQ_HIGHPRI and
WQ_UNBOUND flags cannot be used together.

Change-Id: I4ca4eb6596552866049ea6402e492f463eeaed2c
Acked-by: Baranidharan Muthukumaran <bmuthuku@qti.qualcomm.com>
Signed-off-by: Zhen Kong <zkong@codeaurora.org>