usb: musb: Constify struct musb_fifo_cfg

'struct musb_fifo_cfg' are not modified in these drivers.

Constifying these structures moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  64381	   5537	    202	  70120	  111e8	drivers/usb/musb/musb_core.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  64957	   4929	    202	  70088	  111c8	drivers/usb/musb/musb_core.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/26e6f3e25dc8e785d0034dd7e59918e455563e60.1737234596.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>