pinctrl: at91: fix clk_unprepare and clk_disable order

clk_unprepare shall be called before clk_disable.
Fix the issue by replacing the clk_unprepare and clk_disable calls by a
single clk_disable_unprepare call.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>