watchdog: at32ap700x: Remove redundant platform_set_drvdata()

Commit 0998d063 (device-core: Ensure drvdata = NULL when no
driver is bound) removes the need to set driver data field to
NULL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>