wifi: lib80211: remove unused variables iv32 and iv16

clang with W=1 reports
net/wireless/lib80211_crypt_tkip.c:667:7: error: variable 'iv32'
  set but not used [-Werror,-Wunused-but-set-variable]
                u32 iv32 = tkey->tx_iv32;
                    ^
This variable not used so remove it.
Then remove a similar iv16 variable.
Change the comment because the unmodified value is returned.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230517123310.873023-1-trix@redhat.com
[change commit log wrt. 'length', add comment in the code]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>