ACPI: EC: Restore udelay in poll mode

This fixes keyboard event handling on some systems.

Note that this delay was thought unnecessary, and removed
from linux-2.6.20 with 50c1e11
'ACPI: ec: Drop udelay() from poll mode. Loop by reading status field instead.'

Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/ec.c

@@ -73,6 +73,7 @@ enum ec_event {
 
 #define ACPI_EC_DELAY		500	/* Wait 500ms max. during EC ops */
 #define ACPI_EC_UDELAY_GLK	1000	/* Wait 1ms max. to get global lock */
+#define ACPI_EC_UDELAY		100	/* Wait 100us before polling EC again */
 
 enum {
 	EC_FLAGS_WAIT_GPE = 0,		/* Don't check status until GPE arrives */
@@ -227,6 +228,7 @@ static int acpi_ec_wait(struct acpi_ec *ec, enum ec_event event, int force_poll)
 		while (time_before(jiffies, delay)) {
 			if (acpi_ec_check_status(ec, event))
 				goto end;
+			udelay(ACPI_EC_UDELAY);
 		}
 	}
 	pr_err(PREFIX "acpi_ec_wait timeout,"