net: ipa: use data space for command opcodes

The 64-bit data field in a transaction is not used for commands.
And the opcode array is *only* used for commands.  They're
(currently) the same size; save a little space in the transaction
structure by enclosing the two fields in a union.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>