wifi: brcmsmac: remove more unused data types

Remove unused 'struct brcmu_iovar' and 'struct tx_inst_power'. This
follows commit b2090d93 ("wifi: brcmsmac: remove unused data type").

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230725162400.192357-1-dmantipov@yandex.ru