mfd: ti_am335x_tscadc: Move the driver structure allocation earlier

Allocating the driver structure should be done earlier in the probe so
that we can used its members from the beginning.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-17-miquel.raynal@bootlin.com