crypto/scheduler: fix queue pair configuration

This patch fixes the queue pair configuration for the scheduler PMD.
The queue pairs of a scheduler may have different nb_descriptors sizes,
which was not the case. Also, the maximum available objects in a
queue pair is 1 object smaller than nb_descriptors. This patch fixes
these issues.

Fixes: a783aa634410 ("crypto/scheduler: add packet size based mode")

Signed-off-by: Fan Zhang <roy.fan.zhang@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>