usb: gadget: f_fs: Stop ffs_closed NULL pointer dereference

The struct ffs_data::private_data has a pointer to
ffs_dev stored in it during the ffs_fs_mount() function
however it is not cleared when the ffs_dev is freed
later which causes the ffs_closed function to crash
with "Unable to handle kernel NULL pointer dereference"
error when using the data in ffs_data::private_data.

This clears this pointer during the ffs_free_dev clean
up function.

Signed-off-by: Jim Baxter <jim_baxter@mentor.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>