watchdog: Improve watchdog_dev function documentation

Adjust function comments to the kernel doc format. It
also adjusts some variable names and adds return values.

No functional change.

Changes from V1:
  Change "Returns" to "Return:" (Randy Dunlap)

Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/30378a03e9cd9b5f6e92ec9bf512edc38bad8627.1644589712.git.bristot@kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>