Abstract writes to the service arbiter enable register.

This is in preparation for the introduction of the qat_4xxx driver since
the arbitration enable register differes between QAT GEN2 and QAT GEN4
devices.
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Wojciech Ziemba <wojciech.ziemba@intel.com>
Reviewed-by: Maksim Lukoshkov <maksim.lukoshkov@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>