rtl8180: disable buggy rate fallback mechanism

rtl8180: disable buggy rate fallback mechanism

Currently the driver configures mac80211 to provide two rates for each TX frame:
One initial rate and one alternate fallback rate, each one with its retry count.

HW does not support fully this: rtl8180 doesn't have support for rate scaling at
all, and rtl8185/rtl8187SE supports it in a way that does not fit with mac80211:
The HW does automatically fall back to the next lower rate, and only a lower
limit can be specified, so the HW may TX also on rates in between the two rates
specified by mac80211.  Furthermore only the total TX retry count can be
specified for each packet, while the number of TX attempts before scaling rate
can be configured only globally (not per each packet).

Currently the driver sets the HW auto rate fallback mechanism to quickly scale
rate after a couple of retries, and it uses the alternate rate requested by
mac80211 as fallback limit rate (and it does this even wrongly).

The HW indeed will behave differently than what mac80211 mandates, that is
probably undesirable, and the reported TX retry count may not refer to what
mac80211 thinks, and this could fool mac80211.

This patch makes the driver to declare to mac80211 to support only one rate
configuration for each packet, and it does disable the HW auto rate fallback
mechanism, relying only on SW and letting mac80211 to do all by itself.

This should ensure correct operation and fairness respect to mac80211.
Indeed here tests with iperf do not show significant performance differences.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>