Get rid of some arbitrary API restrictions that limit the kernel
to 64 bridges and 256 ports.

Retain compatibility in GET_PORT_LIST, the existing bridge API
passes 0 for the third argument, and expects 256 entries.

Note: there still is limit of 256 ports due to STP, but this
shouldn't show up in the API, it needs to be handled by the
'add port to bridge ioctl'.