leds: lm36274: use platform device as parent of LED

Instead of registering LED under the MFD device, this driver sets the
parent of the LED it is registering to the parent of the MFD device (the
I2C client device).

Because of this we cannot use devres for LED registration, since it can
result in use-after-free, see commit
a0972fff ("leds: lm36274: fix use-after-free on unbind").

The only other in-tree driver that also registers under the MFD device
(drivers/regulator/lm363x-regulator.c) sets the parent to the MFD
device.

Set the parent of this LED to the MFD device, instead of the I2C client
device.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>