mei: streamline amthif write complete function

Rename the function mei_amthif_irq_process_completed
to mei_amthif_irq_write_complete
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>