usb: gadget: g_ffs: Fix counting of missing_functions

Returning non-zero value from ready callback makes ffs instance
return error from writing strings and enter FFS_CLOSING state.
This means that this this function is not truly ready and
close callback will not be called. This commit fix
ffs_ready_callback() to undo all side effects of this function
in case of error.

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