drivers/iio/adc/at91_adc.c: use clk_prepare_enable and clk_disable_unprepare

drivers/iio/adc/at91_adc.c: use clk_prepare_enable and clk_disable_unprepare

Clk_prepare_enable and clk_disable_unprepare combine clk_prepare and
clk_enable, and clk_disable and clk_unprepare.  They make the code more
concise, and ensure that clk_unprepare is called when clk_enable fails.

A simplified version of the semantic patch that introduces calls to these
functions is as follows: (http://coccinelle.lip6.fr/)

// <smpl>
@@
expression e;
@@

- clk_prepare(e);
- clk_enable(e);
+ clk_prepare_enable(e);

@@
expression e;
@@

- clk_disable(e);
- clk_unprepare(e);
+ clk_disable_unprepare(e);
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>