mei: bus: Synchronous API for the data transmission

Define a truly synchronous API for the bus Tx path by putting all pending
request to the write list and wait for the interrupt tx handler to wake
us up.
The ___mei_cl_send() out path is also slightly reworked to make it look more
like main.c:mei_write().

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