wifi: mt76: mt7996: Fix null-ptr-deref in mt7996_mmio_wed_init()

wifi: mt76: mt7996: Fix null-ptr-deref in mt7996_mmio_wed_init()

devm_ioremap() returns NULL on error. Currently, mt7996_mmio_wed_init()
does not check for this case, which results in a NULL pointer
dereference.

Prevent null pointer dereference in mt7996_mmio_wed_init()

Fixes: 83eafc9251d6 ("wifi: mt76: mt7996: add wed tx support")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Link: https://patch.msgid.link/20250407032349.83360-1-bsdhenrymartin@gmail.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>