mac80211: minstrel_ht_assign_best_tp_rates: remove redundant test

We know this pointer isn't NULL and in fact dereferenced it before,
remove the redundant test.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200326150855.adf551928846.Iae9015573d6c350cc1b12a311d6d13d086beec6c@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>