mt76: mt7615: fix memory leak in mt7615_coredump_work

Similar to the issue fixed in mt7921_coredump_work, fix a possible memory
leak in mt7615_coredump_work routine.

Fixes: d2bf7959 ("mt76: mt7663: introduce coredump support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>