i2c-imc-skylake: downstream A/B lever for the shutdown quiescing

quiesce_on_shutdown (bool, 0644, default on) turns imc_pci_shutdown()
into a no-op when clear, byte for byte the pre-1.0.4 behaviour.  It
exists for the ws1-mint S5-hang investigation: together with furyd's
FURYD_SKIP_CLEAR it lets the serial-console session vary one factor at
a time without rebuilding or reloading.

Downstream only, like the guard branch: a parameter whose only purpose
is A/B-testing one machine is not upstream material.  Keep this commit
out of the mainline v5 respin.

i2c-imc-skylake.c

@@ -148,6 +148,17 @@ module_param(allow_unsafe_access, bool, 0444);
 MODULE_PARM_DESC(allow_unsafe_access,
 		 "allow access without firmware/SMM arbitration (unsafe)");
 
+/*
+ * Runtime-writable so the handback can be A/B tested against the previous
+ * do-nothing shutdown without rebuilding or reloading: with the parameter
+ * clear, imc_pci_shutdown() is a no-op, byte for byte the pre-1.0.4
+ * behaviour.
+ */
+static bool quiesce_on_shutdown = true;
+module_param(quiesce_on_shutdown, bool, 0644);
+MODULE_PARM_DESC(quiesce_on_shutdown,
+		 "hand the engine back to the firmware quiesced at shutdown");
+
 struct imc_chan {
 	unsigned int cmd, data, stat;
 	int idx;			/* channel index 0 or 1 */
@@ -713,6 +724,9 @@ static void imc_pci_shutdown(struct pci_dev *pdev)
 	if (!s)
 		return;
 
+	if (!quiesce_on_shutdown)
+		return;
+
 	/* refuse new transfers, then drain the one possibly in flight */
 	for (i = 0; i < ARRAY_SIZE(s->adap); i++)
 		i2c_mark_adapter_suspended(&s->adap[i]);