mmc: mtk-sd: replace use of system_wq with system_percpu_wq

Currently if a user enqueues a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistency cannot be addressed without refactoring the API.

This patch continues the effort to refactor worqueue APIs, which has begun
with the change introducing new workqueues and a new alloc_workqueue flag:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

Replace system_wq with system_percpu_wq, keeping the same old behavior.
The old wq (system_wq) will be kept for a few release cycles.

Suggested-by: Tejun Heo <[e-mail]>
Signed-off-by: Marco Crivellari <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/mmc/host/mtk-sd.c

···
 	host->data = data;
 	read = data->flags & MMC_DATA_READ;
 
-	mod_delayed_work(system_wq, &host->req_timeout, DAT_TIMEOUT);
+	mod_delayed_work(system_percpu_wq, &host->req_timeout, DAT_TIMEOUT);
 	msdc_dma_setup(host, &host->dma, data);
 	sdr_set_bits(host->base + MSDC_INTEN, data_ints_mask);
 	sdr_set_field(host->base + MSDC_DMA_CTRL, MSDC_DMA_CTRL_START, 1);
···
 	WARN_ON(host->cmd);
 	host->cmd = cmd;
 
-	mod_delayed_work(system_wq, &host->req_timeout, DAT_TIMEOUT);
+	mod_delayed_work(system_percpu_wq, &host->req_timeout, DAT_TIMEOUT);
 	if (!msdc_cmd_is_ready(host, mrq, cmd))
 		return;