[WATCHDOG] w83697ug: add error checking

I noticed the W83697UG driver tries to register a watchdog even though
it already noticed the chip isn't there.

WDT driver for the Winbond(TM) W83697UG/UF Super I/O chip initialising.
w83697ug/uf WDT: No W83697UG/UF could be found
w83697ug/uf WDT: Watchdog already running. Resetting timeout to 60 sec
w83697ug/uf WDT: cannot register miscdev on minor=130 (err=-16)

Patch propagates the error back to wdt_init().

Signed-off-by: Eric Lammerts <eric@lammerts.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>