usb: gadget: f_fs: Do not free IPC log buffer when free instance

Currently we allocate IPC log buffer when instance is created,
and free IPC log buffer when instance is freed.

So we can't get IPC log when issue happens after the instance
is freed.

Move IPC log buffer allocate and free to module init and exit.

Change-Id: Ic8b0f6d91c9bbbf2a6a6ed87f96c04475a09b7f3
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>