usb: f_fs: Free descriptors in func_unbind

Commit 2df1806a ("usb: f_fs: Avoid use-after-free of
ffs_data") added an early return from the ffs_func_unbind
if refcount becomes zero without actually freeing the
descriptors and the eps. Now during next connect, func_bind
fails as the eps are not marked NULL. Fix this by removing
the early return and adding a judgement for refcount to
avoid ffs_data pointer use-after-free.

Change-Id: I95648335bd14ab65909c9598eca3bf72a1cc8d5e
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>