media: i2c: ds90ub960: Fix PLL config for 1200 MHz CSI rate

smatch reports:

drivers/media/i2c/ds90ub960.c:1788 ub960_init_tx_ports() error: uninitialized symbol 'pll_div'.

This is caused by 'pll_div' not being set for 1200 MHz CSI rate. Set the
'pll_div' correctly.

Closes: https://lore.kernel.org/all/8d6daeb1-b62a-bbb2-b840-8759c84f2085@xs4all.nl/

Fixes: afe267f2 ("media: i2c: add DS90UB960 driver")
Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/i2c/ds90ub960.c

@@ -1771,6 +1771,7 @@ static int ub960_init_tx_ports(struct ub960_data *priv)
 		break;
 	case MHZ(1200):
 		speed_select = 1;
+		pll_div = 0x18;
 		break;
 	case MHZ(800):
 		speed_select = 2;