Separate USB code into a file separate from asic/asic_devices.

Separating the USB code from everything else in asic/asic_devices.c goes
a long way toward reducing the use of that file as a dumping ground for
everything that didn't seem to fit anywhere else.
Signed-off-by: David VomLehn <dvomlehn@cisco.com>
To: linux-mips@linux-mips.org
Cc: greg@kroah.com
Cc: linux-usb@vger.kernel.org
Patchwork: http://patchwork.linux-mips.org/patch/1522/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>