wifi: mac80211: fix off-by-one link setting

The convention for find_first_bit() is 0-based, while ffs()
is 1-based, so this is now off-by-one. I cannot reproduce the
gcc-9 problem, but since the -1 is now removed, I'm hoping it
will still avoid the original issue.

Reported-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Fixes: 1d8d4af4 ("wifi: mac80211: avoid u32_encode_bits() warning")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>