wifi: mt76: mt7996: avoid null deref in mt7996_stop_phy()

In mt7996_stop_phy() the mt7996_phy structure is
dereferenced before the null sanity check which could
lead to a null deref.

Fix by moving the dereference operation after the
sanity check.

Fixes: 69d54ce7 ("wifi: mt76: mt7996: switch to single multi-radio wiphy")
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Link: https://patch.msgid.link/20250421111344.11484-1-qasdev00@gmail.com


Signed-off-by: Felix Fietkau <nbd@nbd.name>