mac80211: fix rate mask reset

Coverity reported the strange "if (~...)" condition that's
always true. It suggested that ! was intended instead of ~,
but upon further analysis I'm convinced that what really was
intended was a comparison to 0xff/0xffff (in HT/VHT cases
respectively), since this indicates that all of the rates
are enabled.

Change the comparison accordingly.

I'm guessing this never really mattered because a reset to
not having a rate mask is basically equivalent to having a
mask that enables all rates.

Reported-by: Colin Ian King <colin.king@canonical.com>
Fixes: 2ffbe6d3 ("mac80211: fix and optimize MCS mask handling")
Fixes: b119ad6e

 ("mac80211: add rate mask logic for vht rates")
Reviewed-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20210212112213.36b38078f569.I8546a20c80bc1669058eb453e213630b846e107b@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>