hwmon: (aspeed-pwm-tacho) Switch to using the new API kobj_to_dev()

fixed the following coccicheck:
drivers/hwmon/aspeed-pwm-tacho.c:634:60-61: WARNING opportunity for
kobj_to_dev()
drivers/hwmon/aspeed-pwm-tacho.c:623:60-61: WARNING opportunity for
kobj_to_dev()

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/1609725945-13895-1-git-send-email-tiantao6@hisilicon.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>