mei: fix fasync return value on error

fasync should return a negative value on error
and not poll mask POLLERR.

Cc: <stable@vger.kernel.org> # 4.3+
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>