s390/watchdog: Fix module name in Kconfig help text

The help text for DIAG288_WATCHDOG in drivers/watchdog/Kconfig
still mentioned the misleading old module name vmwatchdog.
This patch changes that to the correct new name diag288_wdt.

Signed-off-by: Philipp Hachtmann <phacht@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>