Currently the for-loop will spin forever if variable supported is
non-zero because supported is never changed.  Fix this by adding in
the missing right shift of supported.

Addresses-Coverity: ("Infinite loop")
Fixes: 48cb3952 ("mac80211: minstrel_ht: improve rate probing for devices with static fallback")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20190822122034.28664-1-colin.king@canonical.comSigned-off-by: Johannes Berg <johannes.berg@intel.com>