hwrng: imx-rngc - use devm_clk_get_enabled

Use the new devm_clk_get_enabled function to get our clock.

We don't have to disable and unprepare the clock ourselves any more in
error paths and in the remove function.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>