rtlwifi: Improve handling of IPv6 packets

Routine rtl_is_special_data() is supposed to identify packets that need to
use a low bit rate so that the probability of successful transmission is
high. The current version has a bug that causes all IPv6 packets to be
labelled as special, with a corresponding low rate of transmission. A
complete fix will be quite intrusive, but until that is available, all
IPv6 packets are identified as regular.

This patch also removes a magic number.

Reported-and-tested-by: Alan Fisher <acf@unixcube.org>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org> [3.18+]
Cc: Alan Fisher <acf@unixcube.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>