wifi: nl80211: use int for band coming from netlink

wifi: nl80211: use int for band coming from netlink

This was pointed out before, but there are issues with just
removing the <0 check since enum representation isn't fixed,
nla_type() returns int but really can only return small
non-negative values, etc. Now newer versions of sparse are
also starting to warn on it. Just use int for the band var.

Link: https://patch.msgid.link/20260316123050.8c2d9f3426a0.I86acfa785982993fbffd148cc59049991bd6158f@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>