wifi: mac80211: remove erroneous sband/link validation

In sta_apply_parameters(), we really no longer need to
check that the link or sband exists, in fact, that's
harmful if link 0 doesn't exist, since then this will
fail.

Just remove this check, it was added for validation of
the sband where used, but it's not used here, it's now
only used in sta_link_apply_parameters() which has an
own lookup and check.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>