video: rk3399_mipi: correct error checking

Pointers are never negative. Use macro IS_ERR() for checking.
cf. rk3288_mipi.c

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>