carl9170: fix WARN_ON triggered by Broadcom HT STAs

Broadcom's Windows driver for the 4313 advertises
an ampdu density of 7 => 16 us. The AR9170 MAC on
the other hand only supports densities up to 8 us.

This patch removes the noisy WARN_ON, because
there is nothing we can do about it.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>