MIPS: Alchemy: Remove USB_DEV_REQ_INT prioritization hack

The Alchemy hardware provides a method to prioritize interrupts
on a controller by assigning them to a differenct core request line.
Assign usb device request interrupt to IC0 Request 0 (which has
highest priority in the core and the dispatcher) and others to
Request 1.  The explicit check for usb device request occurrence
should be obsolete now.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>