usb: mtu3: constify struct debugfs_reg32

mtu3_prb_regs is never changed and can therefore be made const.

This allows the compiler to put it in the text section instead of the
data section.

Before:
   text    data     bss     dec     hex filename
  19966    7120       0   27086    69ce drivers/usb/mtu3/mtu3_debugfs.o

After:
   text    data     bss     dec     hex filename
  20142    6992       0   27134    69fe drivers/usb/mtu3/mtu3_debugfs.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>