mac80211: don't send empty extended rates IE

The association request includes a list of supported data rates.

802.11b: 4 supported rates.
802.11g: 12 (8 + 4) supported rates.
802.11a: 8 supported rates.

The rates tag of the assoc request has room for only 8 rates. In case of
802.11g an extended rate tag is appended. However in net/wireless/mlme.c
an extended (empty) rate tag is also appended if the number of rates is
exact 8. This empty (length=0) extended rates tag causes some APs to
deny association with code 18 (unsupported rates). These APs include my
ZyXEL G-570U, and according to Tomas Winkler som Cisco APs.

'If count == 8' has been used to check for the need for an extended rates
tag. But count would also be equal to 8 if the for loop exited because of
no more supported rates. Therefore a check for count being less than
rates_len would seem more correct.

Thanks to:
 * Dan Williams for newbie guidance
 * Tomas Winkler for confirming the problem

Signed-off-by: Jan-Espen Pettersen <sigsegv@radiotube.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>