clk_prepare()/clk_unprepare() must not be called within atomic context.

This patch calls clk_prepare() once for all from atmel_sha_probe() and
clk_unprepare() from atmel_sha_remove().

Then calls of clk_prepare_enable()/clk_disable_unprepare() were replaced
by calls of clk_enable()/clk_disable().

Cc: stable@vger.kernel.org
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Reported-by: Matthias Mayr <matthias.mayr@student.kit.edu>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>