mei: streamline write complete flow function

Rename the function  _mei_irq_thread_cmpl to
mei_irq_thread_write_complete to make clear it deals
with writing. Remove cl from the parameter list as it
can be extracted from cb block.
Extract the common flow from if statements and document
the logic properly

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>