Core: Fix potential uninitialized var usage in WII_IPC_HLE_Device_usb

If "if (ReadStoredLinkKey->read_all == 1)" is not true, then
num_keys_read would be used uninitialized.
Merge pull request #613 from lioncash/uninitialized

Core: Fix potential uninitialized var usage in WII_IPC_HLE_Device_usb