usb: gadget: net2272 - Correct includes

<linux/irq.h> states:

 * Please do not include this file in generic code.  There is currently
 * no requirement for any architecture to implement anything held
 * within this file.

prefetch() and prefetchw() need <linux/prefetch.h> on m68k:

drivers/usb/gadget/net2272.c: In function ‘net2272_write_fifo’:
drivers/usb/gadget/net2272.c:468: error: implicit declaration of function ‘prefetch’
drivers/usb/gadget/net2272.c: In function ‘net2272_read_fifo’:
drivers/usb/gadget/net2272.c:574: error: implicit declaration of function ‘prefetchw’

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>