usb: gadget: storage: Add error handling for no memory

fsg_common_set_num_buffers() may fail due to ENOMEM. So add
error handling for fail case.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Jaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>