SoC: SOF: ipc: Optimizations for tx message

Merge series from Peter Ujfalusi <peter.ujfalusi@linux.intel.com>:

The series will drop the internal use of 'header' parameter which is always
set to hdr->cmd.

The other simplification is to use the provided message directly as it is
guarantied to be valid throughout the message sending and we can save memory
by not allocating a temporary buffer, also saving on needles memcpy()
operations.