net: netconsole: Standardize variable naming

Update variable names from err to ret in cases where the variable may
return non-error values.

This change facilitates a forthcoming patch that relies on ret being
used consistently to handle return values, regardless of whether they
indicate an error or not.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>