watchdog: shwdt: Remove unused platform_set_drvdata()

This patch removes platform_set_drvdata() which is not used in
the driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>