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.

Fixes: 69d54ce7 ("wifi: mt76: mt7996: switch to single multi-radio wiphy")
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Link: https://patch.msgid.link/20250421112544.13430-1-qasdev00@gmail.com


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