wifi: rt2x00: set VGC gain for both chains of MT7620

Set bbp66 for all chains of the MT7620.

Reported-by: Serge Vasilugin <vasilugin@yandex.ru>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/29e161397e5c9d9399da0fe87d44458aa2b90a78.1663445157.git.daniel@makrotopia.org