[HIFN]: Fix max queue length value

    
All but the last element of the command and result descriptor rings can be
used for crypto requests, fix HIFN_QUEUE_LENGTH.
    
Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>