ACPI: ibm-acpi: do not use / in driver names

ibm-acpi uses sub-device names like ibm/hotkey, which get in the way of
a sysfs conversion.  Fix it to use ibm_hotkey instead.  Thanks to Zhang
Rui for noticing this.
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>

drivers/acpi/ibm_acpi.c

@@ -1854,7 +1854,7 @@ static int __init register_driver(struct ibm_struct *ibm)
 	}
 
 	memset(ibm->driver, 0, sizeof(struct acpi_driver));
-	sprintf(ibm->driver->name, "%s/%s", IBM_NAME, ibm->name);
+	sprintf(ibm->driver->name, "%s_%s", IBM_NAME, ibm->name);
 	ibm->driver->ids = ibm->hid;
 	ibm->driver->ops.add = &ibm_device_add;