can: at91_can: add missing prepare and unprepare of the clock

In order to make the driver work with the common clock framework, this patch
converts the clk_enable()/clk_disable() to
clk_prepare_enable()/clk_disable_unprepare(). While there, add the missing
error handling.

Signed-off-by: David Dueck <davidcdueck@googlemail.com>
Signed-off-by: Anthony Harivel <anthony.harivel@emtrion.de>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>