Improve handling of alternate settings in the USB stack.

Allow setting the alternate interface number to fail when there is only
one alternate setting present, to comply with the USB specification.

Refactor how iface->num_altsetting is computed.

Bump the __FreeBSD_version due to change of core USB structure.

PR: 251856
MFC after: 1 week
Submitted by: Ma, Horse <Shichun.Ma@dell.com>
Sponsored by: Mellanox Technologies // NVIDIA Networking