Revert "char: misc: assign file->private_data in all cases"

This reverts commit 585d98e0

, as it
breaks the FUSE misc driver.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>