media: qcom: camss: clean up a check

Imagine that "->vfe_num" is zero, then the subtraction will underflow to
UINT_MAX.  Plus it's just cleaner to use >= instead.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>