mt76: mt7615: fix typo defining ps work

Fix typo defining ps_work in mt7615_register_ext_phy(). This is not a
real issue since 802.11 power save is not yet support by the external phy

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>