ixgbe: remove awkward typecasts

ixgbe: remove awkward typecasts

The vector/SSE pmd used a different element type for the tx queue sw_ring
entries. This led to lots of typecasts in the code which required specific
use of bracketing, leading to subtle errors.
For example, in the original code:
	txe = (struct ixgbe_tx_entry_v *)&txq->sw_ring[i];
instead needs to be written as:
	txe = &((struct ixgbe_tx_entry_v *)txq->sw_ring)[i];

We can eliminate this problem, by having two software ring pointers in the
structure for the two different element types.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>