netconsole: Make boolean comparison consistent

Convert the current state assignment to use explicit boolean conversion,
making the code more robust and easier to read. This change adds a
double-negation operator to ensure consistent boolean conversion as
suggested by Paolo[1].

This approach aligns with the existing pattern used in
sysdata_cpu_nr_enabled_show().

Link: https://lore.kernel.org/all/7309e760-63b0-4b58-ad33-2fb8db361141@redhat.com/

 [1]
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>