test/event_crypto: set cipher operation in transform

test/event_crypto: set cipher operation in transform

The symmetric session configure callback function in OCTEON TX2 crypto
PMD returns error if the cipher operation is not set to either encrypt
or decrypt. This patch sets the cipher operation for the null cipher
to encrypt.

Fixes: 74449375237f ("test/event_crypto_adapter: fix configuration")
Cc: stable@dpdk.org
Signed-off-by: Ankur Dwivedi <adwivedi@marvell.com>
Acked-by: Abhinandan Gujjar <abhinandan.gujjar@intel.com>