mt76: mt7921: fix kernel warning from cfg80211_calculate_bitrate

mt76: mt7921: fix kernel warning from cfg80211_calculate_bitrate

Fix the kernel warning from cfg80211_calculate_bitrate
due to the legacy rate is not parsed well in the current driver.

Also, zeros struct rate_info before we fill it out to avoid the old value
is kept such as rate->legacy.

[  790.921560] WARNING: CPU: 7 PID: 970 at net/wireless/util.c:1298 cfg80211_calculate_bitrate+0x354/0x35c [cfg80211]
[  790.987738] Hardware name: MediaTek Asurada rev1 board (DT)
[  790.993298] pstate: a0400009 (NzCv daif +PAN -UAO)
[  790.998104] pc : cfg80211_calculate_bitrate+0x354/0x35c [cfg80211]
[  791.004295] lr : cfg80211_calculate_bitrate+0x180/0x35c [cfg80211]
[  791.010462] sp : ffffffc0129c3880
[  791.013765] x29: ffffffc0129c3880 x28: ffffffd38305bea8
[  791.019065] x27: ffffffc0129c3970 x26: 0000000000000013
[  791.024364] x25: 00000000000003ca x24: 000000000000002f
[  791.029664] x23: 00000000000000d0 x22: ffffff8d108bc000
[  791.034964] x21: ffffff8d108bc0d0 x20: ffffffc0129c39a8
[  791.040264] x19: ffffffc0129c39a8 x18: 00000000ffff0a10
[  791.045563] x17: 0000000000000050 x16: 00000000000000ec
[  791.050910] x15: ffffffd3f9ebed9c x14: 0000000000000006
[  791.056211] x13: 00000000000b2eea x12: 0000000000000000
[  791.061511] x11: 00000000ffffffff x10: 0000000000000000
[  791.066811] x9 : 0000000000000000 x8 : 0000000000000000
[  791.072110] x7 : 0000000000000000 x6 : ffffffd3fafa5a7b
[  791.077409] x5 : 0000000000000000 x4 : 0000000000000000
[  791.082708] x3 : 0000000000000000 x2 : 0000000000000000
[  791.088008] x1 : ffffff8d3f79c918 x0 : 0000000000000000
[  791.093308] Call trace:
[  791.095770]  cfg80211_calculate_bitrate+0x354/0x35c [cfg80211]
[  791.101615]  nl80211_put_sta_rate+0x6c/0x2c0 [cfg80211]
[  791.106853]  nl80211_send_station+0x980/0xaa4 [cfg80211]
[  791.112178]  nl80211_get_station+0xb4/0x134 [cfg80211]
[  791.117308]  genl_rcv_msg+0x3a0/0x440
[  791.120960]  netlink_rcv_skb+0xcc/0x118
[  791.124785]  genl_rcv+0x34/0x48
[  791.127916]  netlink_unicast+0x144/0x1dc

Fixes: 1c099ab44727 ("mt76: mt7921: add MCU support")
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>