wifi: mt76: mt7915: Fix null-ptr-deref in mt7915_mmio_wed_init()

wifi: mt76: mt7915: Fix null-ptr-deref in mt7915_mmio_wed_init()

devm_ioremap() returns NULL on error. Currently, mt7915_mmio_wed_init()
does not check for this case, which results in a NULL pointer
dereference.

Prevent null pointer dereference in mt7915_mmio_wed_init().

Fixes: 4f831d18d12d ("wifi: mt76: mt7915: enable WED RX support")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
Link: https://patch.msgid.link/20250407061900.85317-1-bsdhenrymartin@gmail.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>