usb: gadget: storage-common: Set FSG_MAX_LUNS to 16

Mass storage spec allows up to 16 LUNs, so let's not
add some more restrictive limits.

Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>