iio: adc: meson-saradc: check for devm_kasprintf failure

devm_kasprintf() may return NULL on failure of internal allocation thus
the assignments to  init.name  are not safe if not checked. On error
meson_sar_adc_clk_init() returns negative values so -ENOMEM in the
(unlikely) failure case of devm_kasprintf() should be fine here.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Fixes: 3adbf342 ("iio: adc: add a driver for the SAR ADC found in Amlogic Meson SoCs")
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>