musb: fix prefetch build failure

After the prefetch/list.h restructure, drivers need to explicitly include
linux/prefetch.h in order to use the prefetch() function.  Otherwise, the
current driver fails to build:

drivers/usb/musb/musb_core.c: In function 'musb_write_fifo':
drivers/usb/musb/musb_core.c:219: error: implicit declaration of function 'prefetch'
make[3]: *** [drivers/usb/musb/musb_core.o] Error 1

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>