ACPI: Fix a warning of discarding qualifiers from pointer target type

drivers/acpi/ec.c: In function `acpi_ec_ecdt_probe':
drivers/acpi/ec.c:873: warning: passing arg 1 of `acpi_get_devices' discards qualifiers from pointer target type

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/include/acpi/acstruct.h b/include/acpi/acstruct.h
index aeb4498..8848265 100644
--- a/include/acpi/acstruct.h
+++ b/include/acpi/acstruct.h
@@ -146,7 +146,7 @@
 struct acpi_get_devices_info {
 	acpi_walk_callback user_function;
 	void *context;
-	char *hid;
+	const char *hid;
 };
 
 union acpi_aml_operands {