mei: consolidate repeating code in mei_cl_irq_read_msg

A cb is always placed on the completion queue
during discarding queue. Hence this can be performed
upon discard.

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