ath9k: fix RTS/CTS handling

The Tx DMA descriptor has two kinds of flags that select RTS/CTS usage.
The first one (global for the frame) selects whether RTS/CTS or
CTS-to-self should be used, the second one enables RTS/CTS or
CTS-to-self usage for an individual multi-rate-retry entry.
Previously the code preparing the descriptor only enabled the global
flag, if the first MRR series selected the local one.
Fix this by enabling the global flag if any of the MRR entries need it.
With this patch, rate control can properly select the use of RTS/CTS
for all MRR entries except the first one, which is the default behavior.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>