USB: android: Fix error handling when ffs_func_bind fails

In some cases it is observed that usb_add_function() fails with
ffs_function_bind_config() due to ffs_func_bind() fails. When
adbd closes eps, it is trying to reference NULL function pointer
as below:
  __fput
     |
ffs_epfile_release
     |
ffs_data_closed
     |
ffs_data_clear
     |
functionfs_closed_callback
     |
usb_put_function()

Hence handle error case and add check with config->func against NULL.

CRs-Fixed: 848903
Change-Id: Idd260655b8944bd6b21f7bbe3c0340af88f99dfd
Signed-off-by: Mayank Rana <mrana@codeaurora.org>