mei: make mei_write_message more readable

1. reduce hairy casting
2. replace open code with mei_count_empty_write_slots
4. include header size in check for overflow
3. use concise for loop instead of a while loop

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