media: imx: imx7_mipi_csis: Turn csi_state irq field into local variable

The irq field of the csi_state structure is only used in
mipi_csis_probe(). Turn it into a local variable.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>