Instead of complicated logic about when this memory is freed, always free
it during device release(). All the cache pointers start out as NULL, so
it is safe to call this before the cache is initialized.

This makes for a simpler error unwind flow, and a simpler understanding of
the lifetime of the memory allocations inside the struct ib_device.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>