mfd: ti_am335x_tscadc: Don't search the tree for our clock

There is a single clock available in our node, which is named
"fck". The clock handler then points to adc_tsc_fck but no need to point
directly to it and do a full tree search.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-15-miquel.raynal@bootlin.com