staging/mei: remove BUG_ON for testing of the response buffer size overflow

We can remove BUG_ON in mei_irq_thread_read_client_message()
as the testing for response buffer size overflow has already
graceful handling in place.

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