usb: f_fs: Fix ffs_log usage before ffs is set

In order to avoid use-after-free the ffs_data struct
is only set after spin lock since upstream commit
32048f4be071f9a6966744243f1786f45bb22dc2 "usb: f_fs: Fix use-after-free for epfile".
ffs_log tries to access ffs->ipc_log, which now fails
because ffs is not set. Move the ffs_log usage behind
the initialization of ffs to fix this issue.

Change-Id: I471b7fa0116b4ddefd24e4413164923821dd7802