dpdk: fix wrong tx ring size calculations

dpdk: fix wrong tx ring size calculations

At many places code was using constant ring size od 4096
which was defined in macro DPDK_TX_RING_SIZE.

As we support differnet ring size and default value s now
1024, we need to remove DPDK_TX_RING_SIZE and use
value stored in device structure.

For that reason dpdk_device_t.nb_tx_desc is moved to
first cacheline.

Change-Id: I2c2ac11f0f5e8ae779d34f9a9104eaf2921ec34c
Signed-off-by: Damjan Marion <damarion@cisco.com>