wifi: mt76: mt7996: avoid NULL pointer dereference in mt7996_set_monitor()

In the Linux kernel, the following vulnerability has been resolved: wifi: mt76: mt7996: avoid NULL pointer dereference in mt7996_set_monitor() The function mt7996_set_monitor() dereferences phy before the NULL sanity check. Fix this to avoid NULL pointer dereference by moving the dereference after the check.