crypto: omap-aes - Fix configuring of AES mode

AES_CTRL_REG is used to configure AES mode. Before configuring
any mode we need to make sure all other modes are reset or else
driver will misbehave. So mask all modes before configuring
any AES mode.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>