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

In the Linux kernel, the following vulnerability has been resolved: 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().