usb: gadget: mass_storage: Free buffers if create lun fails

Creation of LUN 0 may fail (for example due to ENOMEM).
As fsg_common_set_num_buffers() does some memory allocation
we should free it before it becomes unavailable.

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