misc: mei: fix stalled read

This bug caused severe connectivity issue in the LMS application
(LMS is described in  Documentation/misc-devices/mei/mei.txt)

The bug was introduced in patch:
commit 1ccb7b6249f9bc50678e2a383084ed0a34cc9239
staging/mei: propagate error codes up in the write flow

The patch has reverted the return value logic of some fo function but
the conditional in _mei_irq_thread_read function was not swapped
making read always entering the error path

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