USB: f_fs: Fix use-after-free when unbind races with epfile_io()

EPs freed as part of unbind could be accessed by ffs_epfile_io().
Fix this by moving ffs_log() under disconnect or composition
switch check for similar race with data transfer.

Change-Id: I3ba268577541bd60ec915094fa4f6898d20706ad
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>