wifi: mt76: handle failure of vzalloc in mt7615_coredump_work

vzalloc may fails, dump might be null and will cause
illegal address access later.

Link: https://lore.kernel.org/all/Y%2Fy5Asxw3T3m4jCw@lore-desk


Fixes: d2bf7959 ("mt76: mt7663: introduce coredump support")
Signed-off-by: Kang Chen <void0red@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>