Refactor struct ehci_regs to avoid accessing beyond the end of
port_status. This change results in no difference in the final
object code.

Avoids several warnings when building with -Warray-bounds:

drivers/usb/host/ehci-brcm.c: In function 'ehci_brcm_reset':
drivers/usb/host/ehci-brcm.c:113:32: warning: array subscript 16 is above array bounds of 'u32[15]' {aka 'unsigned int[15]'} [-Warray-bounds]
  113 |  ehci_writel(ehci, 0x00800040, &ehci->regs->port_status[0x10]);
      |                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/usb/host/ehci.h:274,
                 from drivers/usb/host/ehci-brcm.c:15:
./include/linux/usb/ehci_def.h:132:7: note: while referencing 'port_status'
  132 |   u32 port_status[HCS_N_PORTS_MAX];
      |       ^~~~~~~~~~~

Note that the documentation around this proprietary register was
confusing. If "USB_EHCI_INSNREG00" is at port_status[0x0f], its offset
would be 0x80 (not 0x90). The comments have been adjusted to fix this
apparent typo.

Fixes: 9df23151 ("usb: ehci: Add new EHCI driver for Broadcom STB SoC's")
Cc: Al Cooper <alcooperx@gmail.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: bcm-kernel-feedback-list@broadcom.com
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210818173018.2259231-3-keescook@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>