wifi: avoid offset calculation on NULL pointer

ieee80211_he_6ghz_oper() can be passed a NULL pointer
and checks for that, but already did the calculation
to inside of it before. Move it after the check.

Signed-off-by: Michael-CY Lee <michael-cy.lee@mediatek.com>
Link: https://lore.kernel.org/r/20231122030237.31276-1-michael-cy.lee@mediatek.com
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>