[WATCHDOG] wdt.c: remove #ifdef CONFIG_WDT_501

Change the wdt.c watchdog driver so that the code is the same for
both the WDT500 as the WDT501-P card. The selection of the card
is now being done via the module parameter: 'type' instead of the
config option CONFIG_WDT_501.

Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>