crypto: img-hash - use devm_clk_get_enabled() helpers

Simplify the code by replacing devm_clk_get() and clk_prepare_enable()
with devm_clk_get_enabled(), which also avoids the call to
clk_disable_unprepare().

Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>