USB: f_fs: Set ffs->func to NULL after disabling endpoint in set_alt()

When adb root is performed, userspace will close and open ffs_epsfile.
Closing this file will call ffs_functionfs_callback() which does call
remove_config(). This will call ffs_function_eps_disable to disable
endpoints and then calls ffs_func_unbind(). Unbind() will also call
endpoint disable which might lead to disabling endpoint which is already
disabled. Hence set ffs->func to NULL after disabling endpoints in
set_alt().

CRs-Fixed: 557532
Change-Id: I3052bdee74a1793d4e003de4b991d353e5d699b0
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>