mt76x2: init: fix rx filter default value during init

mt76x2_mac_start writes dev->rxfilter to the hardware. It also happens
during init, before dev->rxfilter is filled with the initval register
value, leading to issues like promisc mode being enabled
unconditionally.

Fix this by reading the default value into dev->rxfilter earlier

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>