usb: Allow up to 7 storage devices

The current limit of 5 is not enough for the driver model USB tests. Really
we should not have a limit but the driver model code still uses the
usb_dev_desc[] array, which has a limit.

Increasing the limit by 2 should not bother anyone. Adjust it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>