staging: media: tegra-video: improve error messages

tegra_vi_channels_alloc() can primarily fail for two reasons:

 1. "ports" node not found
 2. port_num > vi->soc->vi_max_channels

Case 1 prints nothing, case 2 has a dev_err(). The caller [tegra_vi_init()]
has a generic dev_err() on any failure. This mean that in case 2 we print
two messages, and in case 1 we only print a generic message.

Remove the generic message and add a specific message when case 1 happens,
so that we always have one specific message without even increasing the
number of dev_dbg*() calls.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>