usb: Define USB_MAX_STOR_DEV in only one place

We can define this in the header file and use it in usb_storage.c. There is
no need to define it twice. Remove the #define from usb_storage.c.

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