media: staging: intel-ipu3: fix unsigned comparison with < 0

The comparison css->pipes[pipe].bindex < 0 is always false because
bindex is an unsigned int.  Fix this by using a signed integer for
the comparison.

Detected by CoverityScan, CID#1476023 ("Unsigned compared against 0")

Fixes: f5f2e427 ("media: staging/intel-ipu3: Add css pipeline programming")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>