MIPS: OCTEON: octeon-usb: use bitfields for control register

Code needs to compile for all platforms in order to move it to
drivers/usb/dwc3. Use Linux standard bitfield access macros
to manipulate control register.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>