coccinelle: irqf_oneshot: reduce the severity due to false positives

The IRQF_ONESHOT should be present for threaded IRQ using default
primary handler.  However intetrupt of many child devices, e.g. children
of MFD, is nested thus the IRQF_ONESHOT is not needed.  The coccinelle
message about error misleads submitters and reviewers about the severity
of the issue, so make it a warning and mention possible false positive.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>