wifi: mt76: mt7921/mt7925: fix NULL dereference in CSA beacon

In the Linux kernel, the following vulnerability has been resolved:

wifi: mt76: mt7921/mt7925: fix NULL dereference in CSA beacon

This patch is based on a BUG as reported by Bongani Hlope at
https://lore.kernel.org/all/20260502125824.425d7159@bongani-mini.home.org.za/

When a channel-switch announcement (CSA) beacon is received,
cfg80211 queues a wiphy work item that eventually calls
mt7921_channel_switch_rx_beacon(). If the station disconnects
(or the channel context is otherwise torn down) between the
time the work is queued and the time it runs, the driver's
dev->new_ctx pointer can already have been cleared to NULL.
mt7921_channel_switch_rx_beacon() then dereferences new_ctx
unconditionally, triggering a NULL pointer dereference at
address 0x0:

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:mt7921_channel_switch_rx_beacon+0x1f/0x100 [mt7921_common]

The same missing guard exists in mt7925_channel_switch_rx_beacon(),
which shares the same code pattern introduced by the same commit.

Add an early-return NULL check for dev->new_ctx in both
mt7921_channel_switch_rx_beacon() and
mt7925_channel_switch_rx_beacon(). When new_ctx is NULL there is
no pending channel switch to process, so returning immediately is
the correct and safe action.

Oops-Analysis: http://oops.fenrus.org/reports/lkml/20260502125824.425d7159@bongani-mini.home.org.za/report.html