usb: storage: add a macro for the upper limit of max LUN

usb: storage: add a macro for the upper limit of max LUN

The meaning of this value is already used in several places,
but with constant values and comments to explain it separately.
It's better to have a central place to do this then use the macro
in those places for better readability.

Signed-off-by: Dingyan Li <18500469033@163.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/20241030083858.46907-1-18500469033@163.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>