wifi: mac80211: avoid u32_encode_bits() warning

wifi: mac80211: avoid u32_encode_bits() warning

gcc-9 triggers a false-postive warning in ieee80211_mlo_multicast_tx()
for u32_encode_bits(ffs(links) - 1, ...), since ffs() can return zero
on an empty bitmask, and the negative argument to u32_encode_bits()
is then out of range:

In file included from include/linux/ieee80211.h:21,
                 from include/net/cfg80211.h:23,
                 from net/mac80211/tx.c:23:
In function 'u32_encode_bits',
    inlined from 'ieee80211_mlo_multicast_tx' at net/mac80211/tx.c:4437:17,
    inlined from 'ieee80211_subif_start_xmit' at net/mac80211/tx.c:4485:3:
include/linux/bitfield.h:177:3: error: call to '__field_overflow' declared with attribute error: value doesn't fit into mask
  177 |   __field_overflow();     \
      |   ^~~~~~~~~~~~~~~~~~
include/linux/bitfield.h:197:2: note: in expansion of macro '____MAKE_OP'
  197 |  ____MAKE_OP(u##size,u##size,,)
      |  ^~~~~~~~~~~
include/linux/bitfield.h:200:1: note: in expansion of macro '__MAKE_OP'
  200 | __MAKE_OP(32)
      | ^~~~~~~~~

Newer compiler versions do not cause problems with the zero argument
because they do not consider this a __builtin_constant_p().
It's also harmless since the hweight16() check already guarantees
that this cannot be 0.

Replace the ffs() with an equivalent find_first_bit() check that
matches the later for_each_set_bit() style and avoids the warning.

Fixes: 963d0e8d08d9 ("wifi: mac80211: optionally implement MLO multicast TX")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230214132025.1532147-1-arnd@kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>