mac80211: fix a crash in minstrel_ht in HT mode with no supported MCS rates

When a client connects in HT mode but does not provide any valid MCS
rates, the function that finds the next sample rate gets stuck in an
infinite loop.
Fix this by falling back to legacy rates if no usable MCS rates are found.

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