iio: Aspeed ADC - Handle return value of clk_prepare_enable

clk_prepare_enable() can fail here and we must check its return value.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Rick Altherr <raltherr@google.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>