mhi: core: Factorize mhi queuing

Instead of duplicating queuing procedure in mhi_queue_dma(),
mhi_queue_buf() and mhi_queue_skb(), add a new generic mhi_queue()
as common helper.

Note that the unified mhi_queue align pm_lock locking on mhi_queue_buf
behavior, taking it with irqsave variant (vs _bh for former queue_skb
and queue_dma version).

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>