staging: iio: lpc32xx_adc: Use devm_* APIs

devm_* APIs are device managed and make code simpler.
This also fixes an error in return type during clk_get
failure.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Roland Stigge <stigge@antcom.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>