usb: gadget: f_mass_storage: Fix the logic to iterate all common->luns

It is wrong to do --i in the for loop.

Fixes: dd02ea5a ("usb: gadget: mass_storage: Use static array for luns")
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>