iavf: use libie_aq_str

There is no need to store the err string in hw->err_str. Simplify it and
use common helper. hw->err_str is still used for other purpouse.

It should be marked that previously for unknown error the numeric value
was passed as a string. Now the "LIBIE_AQ_RC_UNKNOWN" is used for such
cases.

Add libie_aminq module in iavf Kconfig.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Rafal Romanowski <rafal.romanowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>