usb(4): Code refactoring as a pre-step for adding missing synchronization mechanism.

Move code in switch cases into own functions to make later changes easier to track.

No functional change, except for removing a superfluous break statement when
range checking USB_FS_MAX_FRAMES, in the USB_FS_OPEN case.
It should not have been there at all.

Suggested by: emaste@
MFC after: 1 week
Sponsored by: NVIDIA Networking