v4l-utils/contrib/gconv: fix conversion errors on ARIB-STD-B24 strings

Some ARIB strings in operation requires that
codepoints from the 'extra symbols' charset fallback to 'kanji'.
Remove the range check for 'extra symbols' to accept those strings.

Signed-off-by: Akihiro Tsukada <tskd08@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>