sif: rq: Use a workqueue to handle sif_flush_rq

sif: rq: Use a workqueue to handle sif_flush_rq

Orabug: 23491094

In sif_flush_rq, one of the required steps is to acquire
the qp mutex for qp state transition. Thus, this commit
moves the sif_flush_rq into a seperate singlethreaded
workqueue to ensure that sif_flush_rq is safe to call
from any context, including the interrupt context.

Signed-off-by: Wei Lin Guay <wei.lin.guay@oracle.com>
Reviewed-by: Knut Omang <knut.omang@oracle.com>