mei: revamp mei_irq_read_client_message function

1. Rename the function and change parameters order,
 so that first parameter is mei_device
2. Simplify the function code flow
3. Rename helper functions to more self descriptive names
4. Use helpers common functions where possible

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