mt76: fix uninitialized mutex access setting rts threshold

Fix following crash due to a leftover uninitialized mutex access
in mt76x2_set_rts_threshold routine.

[   31.018059] Call Trace:
[   31.018341]  register_lock_class+0x51f/0x530
[   31.018828]  __lock_acquire+0x6c/0x1580
[   31.019247]  lock_acquire+0x88/0x120
[   31.021089]  __mutex_lock+0x4a/0x4f0
[   31.023343]  mt76x2_set_rts_threshold+0x28/0x50
[   31.023831]  ieee80211_set_wiphy_params+0x16d/0x4e0
[   31.024344]  nl80211_set_wiphy+0x72b/0xbc0
[   31.024781]  genl_family_rcv_msg+0x192/0x3a0
[   31.025233]  genl_rcv_msg+0x42/0x89
[   31.026079]  netlink_rcv_skb+0x38/0x100
[   31.026475]  genl_rcv+0x1f/0x30
[   31.026804]  netlink_unicast+0x19c/0x250
[   31.027212]  netlink_sendmsg+0x1ed/0x390
[   31.027615]  sock_sendmsg+0x31/0x40
[   31.027973]  ___sys_sendmsg+0x23c/0x280
[   31.030414]  __sys_sendmsg+0x42/0x80
[   31.030783]  do_syscall_64+0x4a/0x170
[   31.031160]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
[   31.031677] RIP: 0033:0x7f3498b39ba7
[   31.033953] RSP: 002b:00007fffe19675b8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
[   31.034883] RAX: ffffffffffffffda RBX: 00000000012d5350 RCX: 00007f3498b39ba7
[   31.035756] RDX: 0000000000000000 RSI: 00007fffe19675f0 RDI: 0000000000000003
[   31.036587] RBP: 00000000012da740 R08: 0000000000000002 R09: 0000000000000000
[   31.037422] R10: 0000000000000006 R11: 0000000000000246 R12: 00000000012da880
[   31.038252] R13: 00007fffe19675f0 R14: 00007fffe19678c0 R15: 00000000012da880

Fixes: 108a4861 ("mt76: create new mt76x02-lib module for common mt76x{0,2} code")
Reported-by:  <lorenzo.trisolini@fluidmesh.com>
Reported-by:  <luca.bisti@fluidmesh.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>