i2c: rcar: WARN about spurious irqs

The FIXME is very old and probably needed because of some driver bug
like insufficient initialization. It may well be that it was fixed
meanwhile but we never know because the spurious irq is silently
ignored. Add now a call trace when this happens so we have more
information in case the issue still exists.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>