Documentation: qat: rewrite description

The sysfs description contains redundancy on returned and allowed values
list, due to the described sysfs is read-write. Rewrite.

Acked-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Cc: Adam Guerin <adam.guerin@intel.com>
Cc: Tomasz Kowallik <tomaszx.kowalik@intel.com>
Cc: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Cc: Wojciech Ziemba <wojciech.ziemba@intel.com>
Cc: Fiona Trahe <fiona.trahe@intel.com>
Cc: linux-crypto@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>