staging: mei: fix possible usage of uninitialized variable

Fix warning: reported in http://marc.info/?l=linux-kernel&m=130812960402606&w=2



drivers/staging/mei/interrupt.c: warning: 'buffer' may be used uninitialized in this function:  => 198

It is a real issue and wrong path in execution is taken when
list is empty or (cl && _mei_irq_thread_state_ok(cl, mei_hdr))
evaluates to false

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