media: imx: imx7_mipi_csis: Make csi_state num_clocks field unsigned

The num_clocks field of the csi_state only stores positive values, make
it unsigned.

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>