mfd: mxs-lradc: Fix error handling in mxs_lradc_probe()

There is the only path, where mxs_lradc_probe() leaves clk undisabled,
since it does return instead of goto err_clk.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Lee Jones <lee.jones@linaro.org>