crypto/aesni-mb: improve security instance setup

crypto/aesni-mb: improve security instance setup

This patch makes some improvements to the security instance setup for
the AESNI-MB PMD, as follows:
- fix potential memory leak where the security instance was not freed if
  an error occurred later in the device creation
- tidy-up security instance initialization code by moving it all,
  including enabling the RTE_CRYPTODEV_FF_SECURITY feature, into one
  '#ifdef AESNI_MB_DOCSIS_SEC_ENABLED' block

Fixes: fda5216fba55 ("crypto/aesni_mb: support DOCSIS protocol")

Signed-off-by: David Coyle <david.coyle@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>