media: i2c: ds90ub960: fix error handling in ub960_rxport_add_serializer()

Smatch warns:
 drivers/media/i2c/ds90ub960.c:1671 ub960_rxport_add_serializer():
 err: 'rxport->ser.client' dereferencing possible ERR_PTR()

i2c_new_client_device() returns error pointers on failure and in
dev_dbg statement we are dereferencing error pointer which is a bug.

Fix this by using IS_ERR() which checks for error pointers.

Fixes: afe267f2

 ("media: i2c: add DS90UB960 driver")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>