usb: Add support for multiple-LUN mass storage devices

This patch changes `usb_stor_scan' to scan all the LUNs of each mass
storage device.  It also fixes the various commands to correctly set
the LUN field.

Notably, it allows each LUN of GuruPlug's microSD card reader to be
accessed.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>