wifi: rt2x00: set correct TX_SW_CFG1 MAC register for MT7620

Set correct TX_SW_CFG1 MAC register as it is done also in v3 of the
vendor driver[1].

[1]: https://gitlab.com/dm38/padavan-ng/-/blob/master/trunk/proprietary/rt_wifi/rtpci/3.0.X.X/mt76x2/chips/rt6352.c#L531


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/4be38975ce600a34249e12d09a3cb758c6e71071.1663445157.git.daniel@makrotopia.org