watchdog: mt7621_wdt: Remove assignment of dev pointer

Commit 0254e953 ("watchdog: Drop pointer to watchdog device from
struct watchdog_device") removed the dev pointer from struct
watchdog_device, but this driver was still assigning it, leading to
a compilation error:

drivers/watchdog/mt7621_wdt.c: In function 'mt7621_wdt_probe':
drivers/watchdog/mt7621_wdt.c:142:16: error:
	'struct watchdog_device' has no member named 'dev'

Fix this by removing the assignment.

Fixes: 0254e953 ("watchdog: Drop pointer to watchdog device ...")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # v4.5+
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>