usb: isp1760-hcd: Fix fall-through annotations

usb: isp1760-hcd: Fix fall-through annotations

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warning:

drivers/usb/isp1760/isp1760-hcd.c: In function ‘collect_qtds’:
drivers/usb/isp1760/isp1760-hcd.c:788:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
      mem_reads8(hcd->regs, qtd->payload_addr,
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
        qtd->data_buffer,
        ~~~~~~~~~~~~~~~~~
        qtd->actual_length);
        ~~~~~~~~~~~~~~~~~~~
drivers/usb/isp1760/isp1760-hcd.c:792:5: note: here
     case OUT_PID:
     ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comments are modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>