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

In the Linux kernel, the following vulnerability has been resolved:

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()