wifi: mt76: mt7996: fix fortify warning

Copy cck and ofdm separately in order to avoid __read_overflow2_field
warning.

Fixes: f75e4779

 ("wifi: mt76: mt7996: add txpower setting support")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20240203132446.54790-1-nbd@nbd.name