crypto: crypto4xx - Perform read/modify/write on device control register

The Security function on the AMCC SoCs has multiple engines within a
single MMIO range.  The crypto driver currently enables the 3DES
functionality by doing a blind write to the device control register.
This can unintentionally disable other functions like the PKA or TRNG
when the driver is loaded.

Perform a read/modify/write to enable the 3DES function instead.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>