media: v4l: vsp1: Fix bru null pointer access

RZ/G2L SoC has only BRS. This patch fixes null pointer access,when only
BRS is enabled.

Fixes: cbb7fa49

("media: v4l: vsp1: Rename BRU to BRx")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>